When President Reagan and his top national security advisers gathered in Los Angeles this week, they were hoping to hear a definitive answer from the secretary of defense on the most important defense question confronting the administration: what to do about the MX missile. A number of them, according to private accounts from White House aides and other participants, went away disappointed.

Defense Secretary Caspar W. Weinberger talked at length about several options, some concerning ways to deploy the MX, others dealing with possible development of a different, so-called "common missile" that could be used by the Navy and Air Force. Yet, after seven months in office, several months of intense review of the controversial multibillion-dollar MX issue and weeks of speculation about what he would recommend, Weinberger did not make any specific recommendations, sources say.

To some of the participants, including some White House aides, an important opportunity to try to focus the thinking of a president and a divided administration on a crucial question had been missed by the one who has been looking at it the longest.

Still, there was a sense of progress mixed with the disappointment. This was the sense that finally a decision by President Reagan on what to do about the MX missile, B1 bomber and other matters was approaching. White House counselor Edwin Meese III stated publicly that these decisions will be made in three to five weeks.

Though Weinberger reportedly did not make any firm recommendations in laying out the options, a number of participants got the impression that he is now leaning toward development of the common missile, something other than the MX that could be used by both the Navy and the Air Force as a future strategic intercontinental ballistic missile.

But these sources say it is increasingly difficult to figure out exactly what it is that Weinberger has in mind. Some White House aides and military officials say privately that the entire MX question desperately needs to be clarified in internal deliberations so that a unified position can be developed before Congress kills the project out of frustration and lack of confidence. Yet Weinberger did not do that, sources claim. The level of discussion presented by the Pentagon at the Monday meeting was not up to what one senior aide called "presidential level."

In recent weeks, it was widely felt throughout the government and reported in the press that Weinberger was favoring the idea of basing the MX aboard a new fleet of transport planes that would carry the missiles aloft for firing and thus hopefully out of the way of incoming Soviet missiles. That idea is still viewed as certain to get money for further research in trying to develop such an aircraft. But it has now faded somewhat as the likely recommendation for actual deployment.

The impression that Weinberger was at one point going to recommend this air-mobile deployment and that the president was prepared to back him up was so strong that some leading members of Congress wrote to the president or warned him personally that Congress would never approve it. Top Air Force officials made their opposition known through Congress and the press.

Now, the impression that Weinberger may be most interested in a common missile is spreading. "To say that there have been marked changes in direction with great frequency is not inaccurate," said one exasperated Pentagon source. "Nobody in this building mentioned air-mobile this week but it sure was popular last week."

To some administration officials and military officers, the idea of a common missile is almost as bad as the idea of an air-borne MX. Like the air-borne MX, the common missile scheme was looked at and rejected in previous administrations for a variety of reasons that critics contend haven't changed. And merely deciding to develop such a missile still doesn't answer the question of where and how to base it on land, the question that has plagued the MX missile from the outset.

Indeed, sources say that Weinberger may be as alone on the common missile, if that is what he currently favors, as on the air-borne scheme. So again the internal situation within the defense community appears to be that of the secretary tilting one way and most other centers of influence tilting another.

Participants in the meetings Monday and Tuesday say that a powerful array of officials, in one way or another, indicated that they support some form of basing the MX on land, a solution that Weinberger, presumably acting upon the campaign statements of Reagan, is trying to avoid.

The Air Force chief of staff, Gen. Lew Allen, supported by the chairman of the Joint Chiefs of Staff, Gen. David Jones, made his pitch for the first time directly to Reagan for the large land-based deployment of MX missiles and thousands of underground shelters in the southwest that the Air Force has favored for several years.

It is this scheme that the president and Weinberger seem most opposed to, though the president reportedly only asked questions at the meeting and did not signal which way he would decide. Allen's pitch was said to be "professional but without much passion," as one source described it, which added to the problem of trying to focus the issue.

But aside from Allen, Secretary of State Alexander M. Haig Jr., Director of the Arms Control and Disarmament Agency Eugene V. Rostow, CIA Director William J. Casey, United Nations Ambassador Jeane J. Kirkpatrick, budget director David A. Stockman and a representative of Vice President Bush are all said to have made an endorsement for some kind of land-basing scheme for the MX at some point in the California deliberations.

The proposal that is attracting most interest is one that was among several offered by a special 15-member panel set up by Weinberger in March to study alternatives for MX basing and headed by a Nobel Prize-winning physicist, Dr. Charles Townes. The scheme involves beginning the MX program with an initial deployment of 100 missiles in 100 shelters in the southwest, something that essentially buys time while more thought is given to the need to expand the system and more development is done on a possible anti-ballistic missile (ABM) system to defend the shelters from Soviet missile attack.

Though the Air Force still officially favors its much larger land-based system, officials say the Air Force has indicated it would back this Townes panel recommendation, which some officers call the "nose under the tent" solution in that it is at least a start on a larger system.

Other options presented at the meeting include such things as more money to develop the "Big Bird" plane to carry MX, research into deploying missiles on the south side of mountains where they would be harder for Soviet missiles coming over the north polar regions to hit and burying missiles in extremely deep silos.

Whatever the president decides, it will be part of what is called a "strategic package" meant to beef up U.S. nuclear retaliatory forces that will include a new bomber, improved command and control, more emphasis on long-range cruise missiles fired from airplanes, accelerated work on the ABM and the Navy's Trident II missile, which could become the common missile.

Reagan made clear during the campaign and afterward that while he favored the MX missile, he was not impressed by the complicated and environmentally controversial plan for basing it in Utah and Nevada. Since then, powerful senatorial friends of the president from those states and the Utah-based Mormon church have opposed the plan.

Some top military and civilian aides believe that Weinberger continues to act on this basic presidential instinct.

The problem with the MX all along has been that there do not seem to be any clear-cut good choices about where to base it. But those who worry that the administration will wind up either losing the program completely to Congress or make the wrong choice strategically believe that a curious combination of factors is also at work that is making the choice even more difficult.

Critics of the way things have unfolded thus far feel there is an unusual situation within the Pentagon, with uncertainty about the amount and kind of advice Weinberger and his deputy, Frank C. Carlucci, are getting from their staffs on such a crucial issue. The bulk of the MX studies have been done by outsiders on the Townes panel, which hasn't been able to agree on what option looks best. The Air Force, whose views are well known, had been excluded from much of the internal debate on other solutions in recent weeks.

The fact that the Air Force favors a plan to deploy 200 missiles in 4,600 shelters throughout the Great Basin in Utah and Nevada that was ultimately settled on by the Carter administration has added a complicating political dimension. Air Force officers are known to feel that the Reagan White House wants to get away from this at least in part because it is identified with Carter. Yet the scheme, they point out, goes back to Republican President Ford and his defense secretary, Donald Rumsfeld.

In a way, military officials say they have "a painful sense of deja vu," meaning that while the land-based system has a Rube Goldberg quality that instinctively turns new government officials away, several past administrations, after wrestling with alternatives, have eventually come back to it.