More than six months after the Reagan administration decided to aid Cambodia's noncommunist resistance fighters openly in a show of support, the money has yet to be delivered because of disagreements with Thailand, according to administration and congressional officials.

The most recent delay was caused by last month's unexpected collapse of Prime Minister Prem Tinsulanond's four-party coalition government in Bangkok, and the scheduling of new elections in July, one year ahead of schedule. "With a caretaker government in Bangkok, things get a little slow," a U.S. official said.

Delivery of the aid also got bogged down in discussions over what role the Thais would play in distributing it, officials said.

Congress last December passed legislation allowing the administration to provide between $1.5 million and $5 million in overt military or economic aid to the noncommunist Khmer People's National Liberation Front, and a smaller noncommunist group headed by Cambodia's former head of state, Prince Norodom Sihanouk. Those two groups are allied in an unlikely coalition with the communist Khmer Rouge, which ruled Cambodia from 1975 until being ousted in the Vietnamese invasion of 1979.

In January, administration officials said they decided to give the resistance fighters $3.5 million in nonlethal aid, including medicine, shortwave radios and batteries, and paramedical training. That aid would be in addition to millions of dollars that have been reportedly funneled to the noncommunists by the CIA through Thailand.

For logistical reasons, direct U.S. assistance to the Cambodian resistance forces based on Thai soil could only be undertaken with Thai cooperation. But Thai authorities have always tried to "maintain a certain level of deniability" to avoid risking a wider war with Vietnam, according to one Capitol Hill source.

The unexpected election has made this a particularly delicate period for the Thai government, U.S. officials said, as Prem is expected to hold on to power but nonetheless would like to avoid handing his opponents any potentially damaging election issue.

The United States has had a similar problem in beginning a $15 million humanitarian aid program for noncommunist Afghan rebels fighting the Soviets from bases in Pakistan. Implementation of the aid program there was delayed by the establishment last December of a civilian government led by Prime Minister Mohammed Khan Junejo.

The parallels include the mutual desire of governments in Bangkok and Islamabad to avoid becoming too publicly associated with resistance movements based on their soil. The similarity of the circumstances points up one inherent difficulty of enforcing the so-called Reagan Doctrine of aiding noncommunist resistance movements in the Third World.