The Reagan administration's Caribbean Basin Initiative, originally promised as an ambitious, clear-eyed development program for Central America and the island nations on the southern flank of the United States, has rapidly bogged down in a mire of misplaced expectations, unsupported rhetoric and ill-planned meetings, Caribbean diplomats say.

Delegates who gathered here this week for the latest regional working conference began leaving yesterday with what one called a "deep mood of pessimism."

"People seem to be able to agree on global abstractions much more easily than they can on a specific program with specific issues," said one representative of the government of Guyana. "We wanted more emphasis on multilateralism, more emphasis on trade and aid, which we are not going to get, and different mechanisms for administering the program. We won't get that either."

Meanwhile, serious strains are showing among the key donors in the plan. Mexico, one of the initiative's four principal donor nations, did not attend the conference. According to organizers of the meeting, Mexico was irritated because Cuba had not been included as a participant.

Since the July meeting in Nassau, the Bahamas, of foreign ministers from the United States, Canada, Venezuela and Mexico that marked the first step in the initiative, Mexico has insisted that Cuba not be excluded from the plan.

As a compromise, the "Nassau Four" agreed that they would basically seek to enhance and increase their bilateral economic relations with the region. Each donor country could then decide whom it would help, but no potential recipient would be ruled out automatically.

By failing to invite Cuba to the working meeting that ended Thursday, however, an automatic exclusion appeared to have been made, and the Mexican delegation, without fanfare or an official announcement, simply stayed away.

Meanwhile, Venezuela and Canada made it clear that they do not plan to increase substantially their already relatively extensive aid to the region.

The recipients are divided as well. The smaller Caribbean islands are always concerned by the large share of aid that goes to the big islands, and fear that if the larger islands can arrange bilateral agreements to their own benefit at the expense of a broader program they will do so.

U.S.-promoted initiatives by private companies in Jamaica, for instance, may be helpful to that island, but they are not planned on such a scale for any other part of the region.

Meanwhile, Central American nations were expected to attend the conference this week, but only Honduras and El Salvador made brief appearances Wednesday. Several of the Caribbean delegates said Central America's "preparation was not yet sufficient" despite a preliminary meeting in Costa Rica last month.

If there is one common point of agreement among the potential recipients, however, it is on the need for continued and greatly increased direct government aid, which, given its current budget restraints, is precisely the topic Washington wants to avoid.

Delegates to the closed meeting generally were reluctant to talk to the press, but in a recent speech Dominican economist Bernardo Vega summed up the region's reservations about President Reagan's emphasis on private investment as an alternative to increased official aid.

"For audacious and permanent private investment, both national and foreign, to exist in the Caribbean, minimum conditions are required to make the economies workable and safe," said Vega. "The current American emphasis on foreign investment as the motor for growth in our economies would only be realistic if we were operating within that framework of economic viability that is absent today.

"Moreover," Vega said, "even when foreign investment is flowing, there is always an area that can only be attended to by the state, and there the support of international aid is indispensable." As examples, Vega cited hydroelectric dams, canals and highways, the basic economic infrastructure of a country.

In this context, the central focus of discussion at this and previous conferences, according to participants, was what exactly the United States is prepared to deliver to poor nations through the initiative.

The Central Americans, for instance, estimate that they will require $20 billion by the end of the decade. The Caribbean countries have no overall figure, but they clearly are looking for major increases in U.S. assistance, which now amounts to about $3 billion a year for the entire region.

The U.S. delegation here, however, was under instructions not to address the direct aid question at all, according to informed sources.

The Washington delegation, headed by Robert Ryan, ambassador-at-large for the Caribbean Basin Initiative, limited its participation to talk of trade and investment in line with the Reagan adminstration's desire to see private businesses become more involved with Third World development while the U.S. government cuts back on its foreign aid budget along with the rest of its appropriations.

But even on the crucial question of trade, Ryan could offer little concrete encouragement.

Sugar, for instance, is a vital export for most of these countries, and the United States is their principal market. But the U.S. Senate this week passed a bill that would effectively raise the import fee on Caribbean sugar by almost 50 percent a pound.

Ryan, various delegates said, could do little more than explain the intricacies of the division of powers in the United States and express his hope that the new legislation would be watered down in the House of Representatives.

Part of the confusion arises from the initiative's origins. A year ago newly elected Jamaican Prime Minister Edward Seaga and Tom Adams, prime minister of Barbados, began promoting what they called a "mini-Marshall plan" for the Caribbean. When Reagan took office in January, receiving Seaga as his first state visitor and rhetorically drawing a line against communist gains in Central America, there were growing expectations that the United States was willing to support such a plan.

But it is difficult to reconcile Reagan's domestic economic austerity program with the kind of initiative the region hopes for. As one U.S. diplomat said recently, "The natural reaction in the States is if you're going to cut my check, why don't you start with those foreigners first?"

So now the Reagan administration finds itself insisting repeatedly to disappointed nations in the area that this initiative was never intended to be a mini-Marshall plan at all.

Instead, while these nations slide toward bankruptcy, their economies wracked by increases in oil prices and the plunge in world prices for virtually all their exports, Ryan's group emphasized gradual fine tuning and small-scale movement toward increasing the amount of liquor and tobacco American tourists can bring back to the United States, slight liberalization of conditions for receiving most-favored-nation status, bilateral investment treaties and modifying details of the general system of preferences, which does not encompass sugar and many of the area's textile exports.

As the U.S. delegation left for visits to nine other recipient countries that hope to receive help from the initiative, a delegate from one of the "Nassau Four" said, "The United States keeps repeating its plans, but its credibility is going down. There have been enough meetings where nothing concrete is said. The next one needs some concrete proof of commitment. The U.S. delegates say they have a lot of rabbits in the hat, but everyone is waiting to see one on the table."