With the deadline for 1986 budget reconciliation legislation looming at the end of next week, some congressional committees, especially in the Republican-controlled Senate, are having trouble meeting spending targets, according to preliminary staff reports.

Other committees, mainly in the Democratic-run House, are claiming questionable savings or compensating for cuts in some programs but extending or expanding others.

The budget problems grow out of the House and Senate agreement two months ago to produce more than $55 billion in deficit reductions. The agreement came at the end of a six-month effort by Congress to come up with a budget resolution for fiscal 1986, which begins Oct. 1.

But behind the budget pact lurks the real effort, now under way, to turn the paper savings into real savings. "We'll get there, more or less, but it won't be easy and it won't be tidy," said a congressional budget official of the effort to make the changes that will produce the projected savings.

The fiscal 1986 budget resolution that was approved in late July simply set targets for spending cuts, with suggestions as to how to achieve them.

As a first step, authorizing committees were given until Sept. 27 to draft "reconciliation" legislation that would alter laws already on the books to achieve about $18 billion in spending cuts and $2 billion in revenue increases next fiscal year, equaling about a third of the savings needed for 1986. The reconciliation process is to produce a total of $67 billion in savings over three years. The rest of the anticipated three-year deficit reduction total of $276 billion would come from annual appropriations bills as they move through Congress.

In dealing with the reconciliation legislation, Senate committees are said in preliminary staff reports to be having at least some problems agreeing on cuts in rural housing, student loans, government pensions and energy-related programs. Difficulties with the huge farm reauthorization bill are also posing problems for agreement on savings from agriculture and food programs.

In the House, the Ways and Means Committee fell short of its target by $2.4 billion, running into objections from the Budget Committee and stalling action on Ways and Means' $19 billion, three-year package of deficit reductions, which had been scheduled for floor debate yesterday.

And appropriations committees in both chambers have yet to nail down allocation of funds within limits set by the budget agreement, leaving open the question of whether some of the money cut from defense may find its way into domestic spending.

But a more common problem in the House appears to be savings that add up on paper but may not produce long-term deficit reductions. The Budget Committee, for instance, has objected to Banking Committee plans to achieve its target by selling off the rural housing loan portfolio rather than cutting the program. The Merchant Marine and Fisheries Committee has plans to escape pressure for Coast Guard user fees by redesignating as a user fee a previously proposed tax for port development.

Another potential problem is a grab-bag of program extensions and reauthorizations that House, but not Senate, committees are proposing to roll into the deficit-reduction legislation.

"Making the desert bloom" is what a House committee staff worker calls the practice under which lawmakers, parched after a five-year drought in spending for domestic programs since President Reagan took office, use spending-cut legislation as a vehicle to perpetuate or even expand favored programs.

One example is the inclusion of nearly $700 million in welfare benefit increases in the Ways and Means spending-cut package. Another is a one-word change in existing law to force, rather than just allow, the Veterans Administration to provide health care to veterans who qualify for it, "probably the strongest commitment to the veteran population in many, many years," said a proud Veterans' Affairs Committee staff member.

Despite the problems, Senate and House budget officials remain cautiously optimistic that the reconciliation targets will be met, at least on paper.

"Basically we're moving along fairly well," said Senate Budget Committee Chairman Pete V. Domenici (R-N.M.). "It's like fitting a size 10 foot in a size 6 shoe, but we're going to make it fit somehow," said Rep. Butler Derrick (D-S.C.), who is riding herd on the reconciliation process for the House Budget Committee.

However, another congressional budget source was less optimistic. "It may be we won't get 100 percent of the savings in all areas," he said, speculating that 75 to 85 percent may be the result.