Mercy Cabrera, an insurance agent with Sunshine Life and Health Advisors, speaks on the phone as she helps a person with information about an insurance policy under the Affordable Care Act at the store setup in the Westland Mall on November 14, 2013 in Hialeah, Florida. - 

There's something of a magic number when it comes to getting young uninsured people sign up for health insurance through the Affordable Care Act: 38.

Thirty-eight percent of uninsured Americans are young adults. So the Obama administration needs 38 percentof those who enroll to be young adults to ensure a balanced risk pool.

The trouble has been getting the attention of that that group, sometimes called the "young invincibles."

Aaron Smith helped found the nonprofit group with the same name, Young Invincibles. They sit down with young people and walk them through their health plan options, but he says that process is taking more time than expected.   

"I think,” says Smith, “there was an assumption that on day one people would just be ready to enroll and sign up. And I think what we've found is actually, there's a long period that's required to talk to people about health insurance.”

Enroll America is another nonprofit that's working with young adults. Its spokeswoman, Jessica Barba Brown, says officials had expected the months of October and November to be more about education than enrollment. “We know that most young people are not going to take that final step of enrolling until we get closer to the deadline,” she says.

That first deadline is December 15. So far, of the states that are reporting sign-up numbers, they're still under that 38 percent young enrollment goal. Barba Brown says the key is to ramp their efforts up in  the weeks just before the deadline.

“When someone says, 'Yeah, that sounds interesting, I'd like to learn more,' we're going to make sure we follow up with them during that time period so they can go and complete that process,” she says.

One key in getting signups may be text messaging, which is "far and away the most popular way of communicating" among the uninsured, says Jed Alpert of the company Mobile Commons, which is working Enroll America.

"And 80 or 90 percent of the people will respond to the messages as they get them," he says, "which is many, many, many times more than in any other media.”

But of course that's just the first step. The text messages gather information from young people. And, they will ultimately get them to the problem-plagued enrollment site. Which will hopefully get them to insurance.