Part of the reason the U.S. has been unable to make much headway against structural unemployment is that the problem is laden with complexities.

It isn't simply - as some would argue - that hard-to-employ workers are malingerers, who merely are refusing to adhere to the traditional American work ethic.

Rather, the problem stems from such factors as changes in the structure of the economy and government policies that tend to encourage persons to shun low-skilled jobs.

There are these considerations:

The increasing proportion of women and teenagers in the work force has heightened competition for low-skilled jobs. And illegal aliens are taking many jobs that otherwise would be available to the structurally unemployed.

The government's ability to spur the economy to create new jobs had been inhibited by inflation. Policy-makers believe they must keep the budget tight to avoid overheating the economy.

There seems to be virtually no public outcry about the structural unemployment problem, except among blacks and some liberals. Most Americans seem content to keep the jobless rate about 5.75 percent.

The past decade's explosion in welfare and unemployment benefits has given low-skilled persons added flexibility to spurn unattractive jobs they previously would have taken.

Recent steep increases in the federal minimum wage and payroll taxes, while needed by some measures, have reduced the willingness of employers to hire low-skilled persons.

The deterioration of the public schools, particularly in large urban areas, has left many young job-applicants inadequatelly equipped for the slots they're seeking. On the other end, too many now have college degrees.

Low-skilled workers themselves often come from backgrounds where they have little opportunity to learn what's expected of them in the marketplace. Counselors say many aren't even aware they're supposed to show up regularly.

Employers are reluctant to become involved in any program that requires much paperwork. As a result, even the best efforts to link manpower programs to private industry have run into snags.

At the same time, policymakers don't really know what kind of manpower programs work best - or even whether to continue the effort at all. And even if they did, Congress is so divided it's difficult to push through any change.

With such a raft of problems, can the nation really hope to solve its structural unemployment problem?

That question is on the minds of Carter administration policymakers and key congressional leaders as the government's manpower programs come up for new scrutiny this year.

The administration, beset by doubts about the effectiveness of the manpower programs, has shelved plans for their rapid expansion. Instead, the White House has proposed limited new moves in two areas:

First, it wants to begin so-called private-sector initiatives - joint ventures with private industry to train the structurally unemployed and find jobs for them.

Second, it is strengthening a variety of experimental programs for dealing with youth unemployment - from stay-in-school work programs to a new version of the 1930s-style Civilian Conservation Corps to give youths "work experience."

But many outside analysts are coming to even more cautions conclusions about the potential for federal manpower programs. Their prescription: Hold the line on existing efforts, while trying to make current programs better.

Interviews with more than two dozen leading manpower specialists suggest a surprising degree of consensus about what needs to be done:

GOALS: Congress should decide what it wants to accomplish with manpower programs and then stick to it.

TARGET GROUPS: Most of the effort should be concentrated on adult workers, not youths - despite the high teenager unemployment rates.

REGIONAL NEEDS: Local manpower programs need to be keyed to regional economic development, so participants are trained for occupations where jobs are available in the locality.

STAFFING: Repeated program changes, some initiated at local levels, mean that local officials may never become skilled in running them.

EXPECTATIONS: The government - and the public - needs to lower its expectations about manpower programs. While some may be effective, experts argue, these programs just aren't going to produce spectacular results because the problems are so tough.

George Johnson, a University of Michgain economist and one of the more optimistic labor market experts, offers this assessment of the potential for federal manpower programs:

Johnson argues that if the government doubled all its current outlays for employment and training programs, it "might" lower the jobless rate in five years by 0.2 to 0.3 percentage points - if the expansion were managed efficiently.

"But," Johnson cautions, "it is unrealistic to expect miracles of these programs."

Yet miracles are still in demand - in part because the needs of the people caught in the structural unemployment vise are so great. As a result, there still is substantial pressure for the government to take action.

Last year's enactment of the Humphrey-Hawkins "full employment" act was one outgrowth of this concern.

Economists are virtually agreed that the law's incredibly ambitious job goals - an unemployment rate of 4 percent by 1983 - would require a minor miracle.

The only way out is if policymakers can find a way to improve the current "tradeoff" between inflation and unemployment. And to many analysts, the best hope of doing that lies in better training programs.

Still, for all the gloom among the experts about past accomplishments - or even the lack of them - no one is suggesting that the game is not worth the candle.

Declares Michael Wachter, a labor market economist at the University of Pennsylvania; "I believe we can learn from past errors and create successful training programs."

Agrees John L. Palmer, a former Brookings Institution manpower specialist: "The idea is to aim at the margin - how to improve what we have. We have to take a much more microapproach - to look at specific projects."

Alan Fechter, National Science Foundation economist, shares that view. "I think the administration is right, by and large, for reining in," Fechter says. "If you start expanding those programs, it's not clear that you won't get diminishing returns."

Many of the experts are skeptical about the administration's new push to involve private industry more heavily in manpower training. Previous efforts in that direction have not fared much better than direct government operations.

"We've already spend an awful lot without knowing what we got into," says one manpower specialist who's been close to the recent administration reappraisal efforts. "We just have to do better from now on."

But even with better administration and a firmer set of goals, there nevertheless is a question whether in the long run the critical need for stability in the manpower programs will be met.

For all its reluctance to move any further this year, the White House has made structural unemployment measures a touchstone of the president's new welfare reform plan, which officials hope to push through for 1983.

And no doubt if the economy takes a downturn, as most analysts now expect, Congress and the president again will consider public-service jobs as a quick - and visible - antirecession device, no matter what the impact on training.

Altogether, the outlook is not promising, despite more than a few success stories of individuals who have managed to lift themselves out of the cycle of sporadic low-paying jobs that had left them poverty-stricken.

But, as one of the manpower experts interviewed for this series puts it, the alternative to trying is "to risk creating a permanent economic and social underclass that will threaten society" - a choice even more expensive than the current set of flaws. CAPTION: Illustration, no caption