America's volunteer Army is attracting more bright recruits than ever, but that good news has obscured a serious and undiminished problem: too many bright soldiers are leaving service after their first hitch, and too many low-aptitude troops are staying in.

About 46 percent of soldiers reenlisting this year scored between the 10th and 30th percentiles in the military's entrance aptitude test, and 67 percent scored in the bottom half. Those scoring below the 10th percentile are not admitted.

The proportion of returning soldiers scoring below 30, or Category IVs, as the Army calls them, has changed little in the last three years and is more than double the Army's goal.

Pentagon officials, while enjoying their best recruiting year since the draft was abolished in 1973, admit to some concern about the quality of people staying in the Army. While stressing that the aptitude test is only one indicator of performance, they acknowledge they want to keep more bright soldiers to train the smart men and women signing up today.

"You cannot promote the good people if you don't have enough good people to promote," said one Army official who asked not to be identified. "The real problem is in the short term, when we will have NCOs non-commissioned officers who are not nearly as bright as the people they're supposed to be leading."

Army officials say the reenlistment record will improve as the available pool of soldiers improves. Many of those signing up for second terms joined the Army in fiscal 1980, when the Army hit rock bottom.

More than half of the Army's recruits that year scored in the 10-to-30 category. So far this year, only 16 percent of the recruits come from that category, ensuring a higher-quality group that the service could encourage to reenlist three years from now.

Experts outside the Army debate whether the problem is short term, and the debate is part of a larger question about the fate of the volunteer Army if the economy improves. Defense Department officials maintain that improved salaries and benefits along with renewed patriotism have sent the Army, in serious trouble a few years ago, into an "upward spiral" that will continue even in prosperity.

"Last year was the best year in history, and this year is a little bit better," Lawrence J. Korb, assistant secretary of defense for manpower, said last week. Korb was speaking about all branches of the service, but Pentagon officials said the Army's success is most remarkable because it had farthest to go.

"The only thing the Army has to hide this year is its pride," Lt. Gen. Maxwell R. Thurman, deputy chief of staff for personnel, told a congressional committee in March. "So if we appear boastful as we describe how much the Army has improved, it is because no one is more pleased than the Army itself."

Aided by targeted mailings and a sophisticated $160 million annual advertising campaign--"We're not much different in terms of our advertising than Procter & Gamble or Ford Motor Co.," an Army spokesman said--the armed forces will meet their recruiting goals months ahead of schedule this year. In addition, about 90 percent of the Army's recruits this year will be high school graduates, according to Korb, compared with 50 percent in 1974.

But the pool of 17- and 18-year-olds will decrease during the 1980s, and an improved economy may lure many of the brightest to other careers. Rep. Les Aspin (D-Wis.), a member of the House Armed Services Committee, noted that recruitment and retention figures have improved despite a 4 percent pay raise last October.

"The implication is that unemployment is driving them into the Army, so we don't have to pay them any more," Aspin said. "If the economy gets better, we're really going to have trouble."

Army statistics show that an upturn in the economy will affect retention in the quality of people who reenlist more than recruitment. Many 17-year-olds will see the Army as their best shot even in the best of times, Army officials say, but a bright soldier who has learned some skills is less likely to remain during good times.

In 1979, when the unemployment rate was relatively low, less than 60 percent of the Army's top scorers reenlisted. Last year, with unemployment more than 9 percent, the proportion rose to 76 percent. The proportion of Category IVs who reenlisted changed less, and remained higher, throughout the years.

"The low-scoring guy is going to stay with you no matter what," a colonel acknowledged. "He's like a puppy dog."

Army officials said they believe, however, that they will not return to the days when sergeants pressured soldiers to reenlist by promising a platoon three days' leave for each soldier who signed up. During fiscal 1982, the Army for the first time rejected 3,000 qualified soldiers who wanted to reenlist. "Reenlistment isn't a right anymore," an Army spokesman said. "It's a privilege."

The Army, after considerable debate, decided not to use entrance exam scores as the sole criterion for deciding who could reenlist, however, since some low scorers make good soldiers.

"My concern is this: that a person not be penalized for a test someone took four years ago," Korb said. "They should be judged for performance."

Still, Army officials admit that soldiers who score high tend to perform better at every job from cook to armor crewman, and they acknowledge that they do not like the "Cat IV" numbers.

The Defense Department will spend or obligate more than $1 billion in cash bonuses this year, according to a General Accounting Office study, to convince the right servicemen to reenlist. Bonuses as large as $20,000, paid partly in a lump sum and partly in installments, are intended to keep critical specialties adequately staffed by qualified servicemen.

"We have not had to compromise in terms of quality," Army spokesman Maj. George H. Stinnett said. "That goes back to the fact that this is a seller's market.