Millions have signed up for new health-care plans under Obamacare. The next trick: keeping them enrolled. With the health-care law's first enrollment period closed, health insurers, policymakers and advocates are pushing to make sure the newly insured become reliable customers for the health plans. That will be crucial to keeping the down the uninsured rate and will help determine how insurers will view the individual market under the sweeping changes made by the Affordable Care Act.