About 24 percent of eligible adults checked out the marketplaces by the end of December, up from 17 percent in October, a survey from The Commonwealth Fund found.
Forty-one percent of those visitors were young adults, aged 19 to 34, a demographic considered key to the long-term success of the health insurance marketplaces created under the Affordable Care Act.
Still, that means three of every four people who may be eligible for coverage stayed on the sidelines.
"We don't know why they didn't visit," said study author Sara Collins, vice president for health care coverage and access at The Commonwealth Fund.
Technical difficulties with the federal HealthCare.gov website "turned out to be a problem," Collins acknowledged. But another potential factor is simply lack of awareness, particularly among poor individuals.
Just 61 percent of lower-income adults -- those who make $28,725 as an individual or $58,875 for a family of four -- were aware of the marketplaces, also known as exchanges, in December. And only half (52 percent) were aware of the financial assistance available to help pay for coverage.
By contrast, 77 percent of people with incomes above 250 percent of the poverty level were aware of the marketplaces, and 72 percent were aware of the financial help that may be available to them.
"What it does underscore is really the need for outreach efforts on the part of both federal and state exchanges and governments to make sure that people are aware of what they're eligible for," Collins said.
Consumers have until March 31 to enroll in marketplace coverage under the Affordable Care Act, and the survey points to signs that many people will need help doing so.
By December, more people found it easier to determine what their premium costs would be and what benefits would be covered. "But you still see people, a lot of people, having difficulty figuring out what their out-of-pocket responsibilities are going to be," she noted.
Also, greater percentages of people in December versus October reported difficulties finding a plan with the type of coverage they need and a plan they could afford.
Still, 59 percent said they were likely to try to enroll in a plan or find out if they are eligible for financial assistance by March 31.
The Commonwealth Fund survey, the second in a series aimed at tracking Americans' experiences with the marketplaces, is based on a national survey of 622 adults aged 19 to 64 conducted between Dec. 11 and 29. The survey included people who are uninsured or who purchased health insurance coverage on the individual market or through the new marketplaces.
Separately, poll results released Thursday by Enroll America, a coalition formed to aid the enrollment effort, found consumers' lack of awareness of financial assistance is the main barrier to enrollment -- not technical problems with HealthCare.gov.
Most (68 percent) have not visited their marketplace, and 69 percent do not know about financial help that might make coverage more affordable, the survey found.
In response, Enroll America said it has added 58 staff members in 11 target states to train more volunteers who will help with outreach to the uninsured.
That nationwide survey of 910 uninsured adults, aged 18 to 64, was conducted between Dec. 12 and 22.
Visit the U.S. Department of Health and Human Services for more on the Affordable Care Act.