According to the CDC data, as of June 16, nearly 11 per cent of people who had sufficient time to get the second dose missed their ideal window.
The number has increased from 8 per cent earlier in the year, but CDC spokesperson Kate Fowlie said the rise was "not unexpected", the Washington Post reported on Saturday.
The second shot is recommended three weeks after the first Pfizer-BioNTech shot or four weeks after the first Moderna shot.
Second doses were considered missed if more than 42 days had passed since the initial jab.
People skip their second shots for a variety of reasons, such as mistakenly believing that they only need one dose to be protected.
Some people also want to avoid the sometimes-unpleasant side effects that come with the second dose, and others simply miss their second appointment and can't or don't reschedule, the report said.
Some people also mistakenly think if they've already had Covid-19, they just need one dose, David Broniatowski, Associate Director for George Washington University's Institute for Data, Democracy and Politics, was quoted as saying.
Others might have got confused with public health officials discussing whether a single jab is enough, theorising that some immunity is better than none, Broniatowski said.
"When you have those sorts of things, people start to rationalise, 'Well maybe it is not necessary to get that second dose', especially when they have other barriers or other concerns," he noted.
But experts stress that completing vaccination will be crucial as the highly transmissible Delta variant of Covid continues its spread, the report said.
As of Saturday, the CDC said 182,109,860 people had received at least one dose while 156,982,549 people are fully vaccinated.
Broniatowski urged health officials to investigate why people aren't getting second doses and shift their public messaging.
"If the science ends up being that we need booster shots, then we're going to need to figure out, 'How do we communicate that?'" he said.
"I think we can learn a lot right now from looking at the reasons why people who don't follow up are not following up."