The full weight of the penalty will not be felt until April, when those who have avoided buying insurance will face penalties of around $700 a person or more. But even then that might not be enough: For the young and healthy who are badly needed to make the exchanges work, it is sometimes cheaper to pay the Internal Revenue Service than an insurance company charging large premiums, with huge deductibles.

“In my experience, the penalty has not been large enough to motivate people to sign up for insurance,” said Christine Speidel, a tax lawyer at Vermont Legal Aid.

Some people do sign up, especially those with low incomes who receive the most generous subsidies, Ms. Speidel said. But others, she said, find that they cannot afford insurance, even with subsidies, so “they grudgingly take the penalty.”

The I.R.S. says that 8.1 million returns included penalty payments for people who went without insurance in 2014, the first year in which most people were required to have coverage. A preliminary report on the latest tax-filing season, tabulating data through April, said that 5.6 million returns included penalties averaging $442 a return for people uninsured in 2015.