The president’s healthcare law sliced America’s uninsured rate down to historic lows by expanding coverage for tens of millions of Americans. At the same time, however, the number of insured people who still lack affordable, robust coverage is rising sharply as more people buy into high-deductible policies.

A new study from the Commonwealth Fund reveals that about 23 percent of Americans with coverage are considered underinsured—up from 12 percent in 2003. That means roughly 31 million Americans who bought health insurance still have trouble affording treatment under their policies.

The researchers at the Commonwealth Fund defined “underinsured” people as having out-of-pocket costs that total 10 percent or more of their annual income, or a deductible that is 5 percent or more of their income. The study concluded that high-deductible policies are likely the culprit behind this massive influx of underinsured people.

The findings are a huge problem for the Obama administration since the entire goal was to expand access to coverage to millions of Americans that they presumably would use instead of delaying treatment. But a handful of recent studies show that even people with health insurance are delaying treatment because they can’t afford it.

Related: High Deductible Plans Have More People Delaying Treatment

A December Gallup Poll showed at least 38 percent of insured, middle-income people, said they had delayed medical treatment because of the cost. “While many Americans have gained insurance, there has been no downturn in the percentage who say they have had to put off needed medical treatment because of cost,” Gallup’s Rebecca Riffkin wrote in a post on the pollster’s website.

The shift toward cost-sharing and high-deductible policies—defined by the Internal Revenue Service as those with annual deductibles of $1,300 or more for individuals and $2,600 for families--is widespread among exchange policies but also employer plans.

The Commonwealth Foundation’s study, unsurprisingly, reveals that low-income people with coverage are about twice as likely to be “underinsured” than people earning more than 200 percent of the poverty line.

Of course, it’s important to note that while affordability continues to be an issue, significantly more people do have health insurance because of the law.