Because of a wrinkle in the Affordable Care Act, Ms. Ramos made too little money to receive federal aid for buying private insurance — and too much to qualify for Medicaid, the government health care program for the poor. But she found a solution.

Encouraged by counselors at a storefront enrollment center here, she took on more work, busing tables at night and, more recently, cooking tamales to sell out of her tiny apartment. By raising her annual income to about $24,000, Ms. Ramos, 39, qualified for a subsidy that enabled her to buy insurance for just $20 a month.

“I want to be covered for my kids,” she said. “I’ll do whatever I need to do.”

Ms. Ramos is one of many low-income, working adults who are caught in what experts call the coverage gap, eligible for neither federal subsidies nor Medicaid because they live in states that have declined to expand Medicaid under the health care law. And like Ms. Ramos, many of those people are taking second jobs or working extra hours to increase their incomes, hoping to become eligible for assistance that will enable them to afford marketplace plans.