Open enrollment season is coming. (Photo by Andrew Harrer/Bloomberg)

With open enrollment for Obamacare coming up Saturday, it's a good time to take stock of the tens of millions of people who still don't have insurance coverage.

An additional 10.3 million people gained health insurance in the first year of expanded coverage under the Affordable Care Act, according to an analysis published in the New England Journal of Medicine this summer. We still won't have the most official count from the U.S. Census Bureau until next fall, but that's the number the Obama administration is using. And that, according to the Department of Health and Human Services, leaves about 32 million people still lacking coverage heading into this ACA open enrollment period.

Here's what we know about who's still uninsured and what's kept them from getting covered.

The big picture

One of the best looks at the uninsured right now comes from the Gallup-Healthways Well-Being Index, which has been tracking the uninsured rate since 2008. The most recent survey from October found that the overall uninsured rate dropped from 18 percent before last year's open enrollment period to 13.4 percent after the sign-up period ended in April. And the rate has remained steady since then.

What about the young invincibles?

There was a lot of attention paid to the young adults in the last enrollment period. They lack insurance more than any other age group, and they're an attractive demographic for insurers to cover because they're relatively healthy. The Gallup survey shows they made coverage gains over the past year, but people between 18 and 34 years old still trail other age groups.

Coverage gains across racial groups

An earlier Gallup poll from June also showed declines in the uninsured rate among whites, blacks and Hispanics. There has been a particularly strong emphasis on enrolling Hispanics, who have the highest rates of uninsurance of any demographic. They're a comparatively younger population, but they also pose enrollment challenges. Gallup showed that their uninsured rate dropped 5.5 percentage points by the end of April.

Geography matters

As expected, the uninsured rate has dropped faster for adults in about half the states that expanded their Medicaid programs, according to the Urban Institute's Health Reform Monitoring Survey. This effect is particularly noticeable in the southern states, which have resisted the Medicaid expansion and is now home to a greater share of uninsured adults. About 4 million people aren't eligible for either Medicaid coverage or the ACA insurance marketplaces in the 23 states that haven't expanded Medicaid.

Slowing progress for kids

The uninsured rate has dropped for pretty much every demographic group this past year — except children, according to another Urban Institute analysis. And it seems that the children's uninsured rate has been leveling off over the past couple of years, according to the Center for Children and Families at Georgetown University. How this rate looks in the future also depends on whether Congress next year reauthorizes the Children's Health Insurance Program, which provides low-cost health coverage to children in families that aren't poor enough to qualify for Medicaid. 

(Georgetown University Health Policy Institute)

Enrollment period? What enrollment period?

As the most recent Kaiser Family Foundation tracking poll shows, it's going to take some extra effort to reach the remaining uninsured. About nine in 10 uninsured adults potentially eligible for marketplace coverage didn't know the enrollment opened again in November. About two-thirds said they knew "only a little" or "nothing at all" about the marketplaces, and just more than half (53 percent) said they didn't know the ACA offers financial assistance to help people purchase insurance.

Why aren't they signing up?

About two-thirds of the uninsured didn't bother looking for coverage in the past enrollment period, according to a report this summer from Enroll America, the national industry-backed group. The most frequent reason people offer for being uninsured — and this is true of basically every survey on the topic — is affordability of coverage. Another Kaiser poll from earlier in the year, just after the first Obamacare enrollment period ended, also showed some confusion about people's obligations to get covered under the law.