WASHINGTON — The federal health-care exchange that opened a dozen days ago is marred by snags beyond the widely publicized computer gridlock that has thwarted Americans trying to buy a health plan. Even when consumers have been able to sign up, insurers sometimes can’t tell who their new customers are because of a separate set of computer defects.

The problems stem from a feature of the online marketplace’s computer system that is designed to send each insurer a daily report listing people who have just enrolled. According to several insurance industry officials, the reports are sometimes confusing and duplicative. In some cases, they show — correctly or not — that the same person enrolled and canceled several times on a single day.

The ability of consumers to sign up for a health plan, and the ability of the insurers to know who they are covering, is key to the success of the federal law that will for the first time require most Americans to buy health insurance starting Jan. 1. The site is the main path for millions of Americans in 36 states to purchase new coverage.

The flawed enrollment reports illustrate that the website www.healthcare.gov is bedeviled by problems that go beyond what the Obama administration has acknowledged in explaining the creaky performance of the exchange so far.

At the White House and the Department of Health and Human Services, officials have portrayed the exchange as a victim of its own popularity, with a larger-than-expected crush of Americans rushing to a website that wasn’t built to accommodate so many people at once.

That explanation puts the focus up front — on the servers and software that help consumers take the first step of registering for an account. Evidence is emerging from the insurance industry and elsewhere, however, that the exchange also has flaws that show up further along in the process — as consumers try to check whether they qualify for federal subsidies and as insurers try to find out who has enrolled.

For instance, one major insurance carrier, Cigna, sent a notice Wednesday to insurance brokers instructing them to wait until November to try to sign up customers who might qualify for a subsidy, according to Joseph Mondy, a Cigna spokesman. He said the company does not yet trust the reliability of the part of the exchange that is supposed to calculate the tax credits that will, for the first time, help some Americans pay for private health coverage.

Cigna is one of the insurers that has built its own online “portal” for brokers to use, but that portal must communicate with the federal exchange to find out about a potential subsidy.

Cigna is selling health plans via the federal exchange in four states: Arizona, Florida, Tennessee and Texas. Mondy said the carrier has seen “multiple enrollments” coming through for the same customer on the same day.

A Blue Cross Blue Shield plan in a southern state said that it has also gotten simultaneous reports of the same consumers enrolling as of Jan. 1 and cancelling as of Dec. 31, 2014.

“It’s a glitch that . . . needs to be fixed,” said a spokesman for the plan, who, like most insurers interviewed, spoke on the condition of anonymity to avoid antagonizing the Obama administration.

Compounding the confusion, these electronic enrollment files are missing a critical element that they were built to include: a time stamp that would let a health plan track whether a consumer’s last step on the site was to actually sign up.

Insurers are uncertain of the cause of the flaws, with some speculating that the exchange cannot consolidate consumers’ moves through the website as they shop for and ultimately choose a health plan. Others say the problem could be unrelated software errors.

Federal health officials declined to discuss the problem with the enrollment reports. “As individual problems are raised by insurers, we work aggressively to address them,” said Brian Cook, a spokesman for the Centers for Medicare and Medicaid Services, the branch of HHS that is overseeing the insurance exchange.

Meanwhile, the government has tweaked the exchange to deal with the initial problem — accessing the heavily used site — to try to make it easier for shoppers to obtain information without going through the sometimes cumbersome process of registering for an account.

Starting Thursday, a new tool points consumers more directly to a list of the health plans in their community and sample prices. Before, this information was on the site only in an Excel spreadsheet that was hard to find and to understand.

Obama administration officials insisted that this was not a major change to the website, because consumers still need to create an account to find out about subsidies and the specific costs of various health plans for people their age.

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