Paul D. Donahue and his wife, Angela, are among more than a million Americans who have signed up for health coverage through the federal insurance exchange. Mr. Donahue has a card in his wallet from his insurer to prove it. But when he tried to use it to get a flu shot and fill prescriptions this week, local pharmacies could not confirm his coverage, so he left without his medications. Similar problems are occurring daily in doctors' offices and drugstores around the country as consumers try to use insurance coverage that took effect on Jan. 1 under the Affordable Care Act.
