There rarely seems to be a time on Capitol Hill when an answer to a farm question is the right answer, so Agriculture Secretary John R. Block has had his share of credibility problems with lawmakers in Congress.

The loudest and most frequently heard complaint about Block is that he kept seeing light at the end of the economic tunnel when legislators saw none.

Many members of Congress now view Block's creation of the payment-in-kind (PIK) crop-swap program to reduce farm surpluses as a tacit confession that the Reagan administration was unduly optimistic and egregiously off target in calculating federal farm-support costs, which could hit $18 billion this year.

Some of Block's credibility problems, in the context of the Washington political funny farm, are surely to be expected. Now, however, some of his minions from the USDA are encountering deeper doubts when they go before congressional committees.

Badgered at home by farmers in financial trouble, farm-state members are increasingly impatient with USDA poobahs who insist that things are not nearly as bad as the press and farmers' complaints would have one believe.

Their frustration is with Frank W. Naylor Jr., undersecretary for small community and rural development, and Farmers Home Administration chief Charles W. Shuman, who keeps saying things that raise legislators' dander. And in return, both are getting their knuckles rapped in rare fashion, even by members of their own party.

A focus of the current unhappiness is FmHA, which lends money to farmers who can't get it anywhere else. Naylor and Shuman deny it, of course, but more and more legislators are subscribing to the idea that FmHA has an unwritten policy of forcing indebted farmers out of business.

This has led to some unusual exchanges. Earlier this month, Sen. James A. McClure (R-Idaho) bluntly told Naylor that the USDA official wasn't telling the truth about FmHA lending policy. McClure had amassed a sheaf of hard-luck stories and examples of FmHA foulups in Idaho that he said put the lie to Naylor's assertions that all was well.

At the same Senate Appropriations subcommittee hearing, Naylor and Shuman matter-of-factly revealed that FmHA has run out of operating-loan money for the current quarter in 17 states and that there is no quick and easy solution in hand. Senators were stunned--even indignant--over the disclosure without formal warning in advance.

Sen. Thad Cochran (R-Miss.), the subcommittee chairman, was among the most distressed. He earlier had heard rumors of the shortfall, but didn't learn of it officially until the hearing. Worse, FmHA had not told him that his home state also has run out of fourth-quarter money, with the fiscal year only half gone.

Less than a month before, Shuman had been before the House Agriculture Committee saying that FmHA had no money problems. Was there any question, Rep. Thomas A. Daschle (D-S.D.) had asked, about FmHA's ability to provide loan money for all who needed it? None whatsoever, Shuman had assured the panel.

MUSICAL CHAIRS . . . . South Dakota voters didn't think enough of his expertise to keep him in Congress, but rancher Clint Roberts has landed at the USDA as a special consultant on international trade matters.