There are some troubling signs. A high proportion of people signing up for new plans are older or have existing health problems—and not enough younger, healthier people may be joining the plans to balance them out and make the plans profitable.
If the trend continues as 2014 enrollment closes at the end of March, insurers in the new marketplaces will think twice about losing money and withdraw plans.
Provisions of the law aim to limit insurers' risk by redistributing money to those with less-healthy customers, but it isn't yet clear how effective they will be.
Some previously insured consumers are vexed that deductibles under the new policies are higher than under prior plans, which can make doctors' visits and procedures more expensive than they anticipated.
The array of challenges has hurt Mr. Obama's standing and public support of the law, which polls show dropped to a new low.
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