Despite billions of dollars invested in helping more students from low-income families go to college, a new analysis shows enrollment by those students has dropped sharply.

Between 2008 and 2013, the percentage of all high school graduates who went straight to college dipped by three percentage points. Among students in the lowest income bracket, enrollment dropped by 10 points, from 56 percent to 46 percent.

“What’s surprising is that other things suggest we should be seeing increases in college participation by low-income students,” said Terry Hartle, who co-authored the report from the American Council on Education, which represents colleges and universities.

High school graduation rates have risen substantially, Hartle said, so the pool of potential low-income college students has grown. Federal grant aid for those students has doubled during the Obama Administration, to nearly $33 billion. Meanwhile, programs to help low-income students prepare for and apply to college have multiplied.

“What we’ve done here is simply identify a deeply troubling, counter-intuitive finding, and I think we need to do some more work to get a better handle on why this is taking place,” Hartle said.

He did suggest some theories. A stronger job market may have lured more low-income students straight into the workforce. Rising tuition prices and questions about the value of a degree may have turned others away. There’s even some chance the data, from the U.S. Census Bureau’s Current Population Survey, are wrong, Hartle said.

“But this is one of the best surveys in the world,” he said. “There’s no reason to expect a data problem.”

Whatever the cause, the findings are troubling, said Dewayne Matthews, vice president for strategy development at the Lumina Foundation, a group that promotes college access.

“All of the evidence that we look at suggests that people who do not complete some level of postsecondary education are going to be significantly disadvantaged throughout their life, both economically and socially,” he said.

Over the longer term, though, enrollment has increased among low-income students, said Julie Ajinkya, director of applied research at the Institute for Higher Education Policy. Low-income students are less likely than others to start college immediately after high school, she said.

Recent efforts, like simplifying the financial aid process and improving college counseling in high school, haven’t had time to show results, Ajinkya said.

“The more that we actually divert and invest resources towards these successful interventions, it’s just a matter of time before we start to see payback,” she said.

Without the huge investment in college access, she and others pointed out, the numbers would likely be worse.