Lindy Wagner, a spokeswoman for Blue Shield of California, said that 80 percent of those who signed up for its plans had paid by the due date, Jan. 15. Blue Shield has about 30 percent of the exchange market in the state.

Matthew N. Wiggin, a spokesman for Aetna, said that about 70 percent of people who signed up for its health plans paid their premiums. For Aetna policies taking effect on Jan. 1, the deadline for payment was Jan. 14, and for products sold by Coventry Health Care, which is now part of Aetna, the deadline was Jan. 17.

Mark T. Bertolini, the chief executive of Aetna, said last week that the company had 135,000 “paid members,” out of 200,000 who began to enroll through the exchanges. “I think people are enrolling in multiple places,” he said in a conference call. “They are shopping. And what happens is that they never really get back on HealthCare.gov to disenroll from plans they prior enrolled in.”…

Obama administration officials said they did not know how many people signing up for coverage had paid their premiums because the government had not finished building the “back end” of the computer systems needed to pay insurers.