Just a few weeks before the third Obamacare enrollment season begins, researchers are pointing out that millions of people are still uninsured, despite the law, and that there are real hurdles to convincing people to sign up.

The first two enrollment seasons made a sizable dent in the U.S. uninsured population, as about 17 million Americans have gained coverage through the Affordable Care Act’s various provisions, the Department of Health and Human Services estimated this week.