In the final stretch of enrolling Americans in ObamaCare, the administration is lagging far behind its own goals -- leaving uncertain whether the program is attracting enough people for the system to work.

The Department of Health and Human Services reported Tuesday that more than 940,000 people signed up in February, bringing the total enrollment number to 4.2 million. That's well short of the unofficial goal of signing up 7 million by the end of open enrollment on March 31.

The numbers renewed calls from Republicans to push off the looming penalty for not buying insurance. And they revived accusations that the administration still is not telling the whole story behind those numbers.

"Given these dismal enrollment numbers, the president needs to work with Congress to get rid of this year's individual mandate penalty," said Brendan Buck, spokesman for House Speaker John Boehner.

Hoping for a surge in the final weeks, administration officials are going to new lengths to promote the law and encourage people to sign up -- including President Obama plugging HealthCare.gov during a YouTube appearance with comedian Zach Galifianakis.

"During this final month of open enrollment our message to the American people is this: you still have time to get covered, but you'll want to sign up today," HHS Secretary Kathleen Sebelius said in a statement on Tuesday.

The administration recently has backed off the goal of enrolling 7 million by month's end. But the government also is coming up short in attracting young adults into the system. The government wanted roughly 40 percent of enrollees to be between 18 and 34 years old; the latest report shows just a quarter of those who have selected a plan are in that coveted age group.

The government, and the insurance industry, wants to attract a high percentage of young adults in order to offset the costs of taking on older, less-healthy customers. Failure to attract enough younger customers could result in higher premiums for everyone else.

It is possible that many participants could be waiting until the final few days and weeks of the month to enroll -- after March 31, most of those who do not have insurance will be required to pay a penalty. But Republicans were skeptical.

"It seems the president's push to enroll young adults is far too little, too late," Buck said. "The administration won't tell us how many people have actually paid for a plan or how many were previously uninsured. But what we do know is that young adults -- those who the White House repeatedly said are critical -- are deciding the health care law is a bad deal. Now, millions stand to be forced to pay a new tax because of this law."

Republicans on the House Energy and Commerce Committee echoed those concerns, and alleged that the president's recent media appearances "suggest that the administration is panicked as the March 31 deadline fast approaches."

Lawmakers say the numbers are still confusing. Though the administration has reported the number of sign-ups, it has not said how many of those individuals have paid their premiums. Further, the administration has not said how many of those enrolling through the ObamaCare exchanges were previously uninsured.

That figure matters because the health care overhaul was originally pitched as a way to cover the nation's estimated 47 million uninsured. But a newly released study suggested many of those people are not being reached. The study by McKinsey & Co. showed that of the uninsured eligible to sign up for an ObamaCare private plan, just 10 percent said they had done so. Further, it found that just a quarter of those who did sign up for coverage in the marketplaces were previously uninsured. That suggests the bulk of those signing up are simply switching from one plan to another.

HHS and supporters of the health care law countered Republicans' skepticism with a more upbeat view.

"Now that over 4.2 million Americans have enrolled in private coverage through the Health Insurance Marketplace, on top of the millions who have received coverage through Medicaid, more and more Americans are gaining the peace of mind and financial security that comes with having health insurance," Anne Filipic, president of Enroll America, said in a statement. "With 83% of consumers receiving financial assistance to help pay for their plans, quality health care has finally become a reality for millions, and we're confident that enrollment will continue to accelerate as we approach the March 31st deadline for coverage."

While millions are joining through the state and federal insurance exchanges, millions more are signing up through Medicaid. In another wrinkle to emerge from the law's implementation, prison inmates are apparently being signed up via the expanded Medicaid as well.

The New York Times reported this week that jails and prisons are taking advantage of the new Medicaid criteria to cover single, childless inmates for certain hospital stays -- in turn transferring the cost from the states to the federal government. Those individuals can also remain covered once they leave jail or prison.