Uncle Sam's check isn't always in the mail.

Last May 24 Edmund Dwyer of Fairfax bought a $10,000 six-month Treasury bill. When his repayment check had not arrived by Dec. 1, he began making inquires at the Treasury's Bureau of Public Debt. At first he was assured the check was in the mail. By Dec. 12, a clerk told him she had found his check voucher on a desk "loaded with undeliverable checks," Dwyer related. It bore an incorrect address. On Dec. 22, nearly a month late, Dwyer received a check for $10,000. He figures that not having his money on time cost him $100 in interest he could have earned had the money been invested at 11 percent, in a money market fund, for example.

On Dec. 13 a Bethesda public health official also bought a $10,000 six-month Treasury bill. Because this security is purchased at a discount, the official expected to receive a check for $595 within the Year had come and gone with no check, he started calling the Treasury, finally reaching an assistant section chief in the book entry accounts division who informed him that a computer tape containing his payment order along with those of 79 other investors had been erased. He was told a manual search of the records would be made.

In the time that it took for his check to arrive, on Jan. 12, the official calculated that the government had benefitted from the interest-free use for almost a month of at least $595 from 80 investors, or $47,600. "They told me I was the first one to complain, that no one else had reported the error. You can be sure that if it was a big institution with a million-dollar investment, they would have noticed the mistake a lot sooner," the official said.

And last week the greatest snafu to date occurred: $6.3 million in discount checks belonging to some 7,060 individuals and a few institutions got "lost" for a week due to a computer breadkown. At even 6 percent interest, that would represent a loss to investors in foregone interest of more than $7,000.

The great paper explosion accompanying the fantastic growth of money market investments has led to complaints by customers about lengthy delays in sending checks, clerical errors and difficulties in opening accounts and making redemptions. Money market mutual funds have been singled out for criticism, but the U.S. Treasury apparently also has its share of glitches.

The lines outside the Bureau of Public Debt in Washington where anxious people wait to put down their $10,000 before 1:30 p.m. on auction days resemble the queues for bettors in front of the 10-cent slot machines in Atlantic City. People with questions or complaints who telephone the bureau listen to the same recorded messages ad nauseam before a live voice responds. Investors now are waiting 12 weeks for certificates for notes and bonds that were once mailed out within two weeks.

Two years ago, 1,100 persons on average bought Treasury securities each week. By October 1978, when the discount (interest) rate on 26-week bills had risen to 8.196 percent, the average weekly number of new accounts was 3,300. Today, with the rate at 11.783 percent, it averages 14,000 or more.

Stated another way, in October 1978 the Treasury had 45,000 active individual accounts totaling $942 million. A year later it had 225,000 accounts totaling $3.4 billion. (These figures include only noncompetitive bids or tenders because more than 99 percent of individual investors accept the rate determined at auction by the competitive bids of large financial institutions.) In the past two weeks, demand by individuals has been very heavy, amounting to 22 percent of total sales, or $2.8 billion.

Treasury securities are sold at 36 Federal Reserve banks across the country, as well as through intermediaries. The regional banks are responsible for opening an account and sending the first discount check. Then the account is sent to Washington where the Bureau of Public Debt's investor accounts division is responsible for processing repayment checks. If the bill is rolled over, or reinvested, the central office takes responsibility for sending out subsequent discount checks.

Although the volume of tenders has more than quadrupled in the past year, the number of personnel in the Treasury responsible for sending out discount and repayment checks has increased by just one-third, to 226, according to Richard Gregg, assistant commissioner for financing in the Bureau of the Public Debt, the office that conducts auctions and distributes the results. "We're constrained in how many people we can hire," he said. "We put in a lot of overtime around here."

Peter Cirafici is one of those working 60 to 70 hours a week to keep up with the paperwork. He was the assistant section chief reached by the Bethesda investor about his missing discount check. He confirmed that the tape containing orders for issuing approximately 80 checks had been "accidently erased or incorrectly processed," but denied there was any danger to investors. Cirafici said that in addition to the repayment tape there was a recording of the original purchase plus the physical document. "There are sufficient safeguards if one aspect of the system fails," he added. "It is impossible that someone wouldn't get paid."

As for Ed Dwyer's missing repayment check, he claimed the Treasury was unwilling to admit its error and blamed him for writing his address indistinctly on his purchase order. But Dwyer pointed out that the discount check, mailed to the address he wrote down, had reached him without delay. Dwyer, who is listed in the telephone directory, also accused the Treasury of not making any effort to trace him until he complained. He said the acting assistant chief of the book entry section, James Lindahl, had pleaded personnel shortgages. Finally, Dwyer's offer to pick up the check in person was refused; it would be remailed.

Lindahl denied he had told Dwyer it was his fault. As for tracing, he said, "We don't sit on return checks and wait for someone to complain; we find out who it belongs to and mail it out." The section's policy is to send out a first class letter notifying the intended recipient the check could not be delivered. This is sometimes followed up by a telephone call and other tracing.

Ron Couch, customer liaison officer in the bureau's division of investor accounts, said the post office returns to the bureau about a dozen undeliverable checks a week, but that there were 43 returns during the last week of November, when 21,000 checks -- including Dwyer's -- were mailed out. Allowing for the holiday mail rush, Couch said Dwyer's check probably had been back at the Treasury for only four working days before his call to Lindahl, "not an unusual time" for the tracing process.

He characterized last week's snafu as "the first time a mass payment has been delayed because of a nondelivery by the disbursing office."

Couch said that because complaints aren't logged, he could not tell the precise number, but that complaints have not increased out of proportion to the number of transactions. That would represent an increase of about 425 percent. Despite increased volume, investors should still expect to receive their discount checks a week or 10 days after issue, Couch added. (The issue date is the Thursday following the Monday -- or sometimes Friday -- auction.) Repayment should be expected on the due date.