Although presiding over major national challenges such as warfare and financial crisis, one of President Obama’s most vexing troubles appears to be a malfunctioning and mercurial website:

Unable to stop implementation of the Affordable Care Act of 2010, whose major provision requiring Americans to purchase healthcare insurance — or face a “tax” — was upheld by the U.S. Supreme Court last year, Congressional Republicans continue to criticize the administration for delays in the website rollout.

On Monday, U.S. Secretary of Health and Human Services Kathleen Sebelius announced she would testify next week before the U.S. House Energy and Commerce Committee, after ducking repeated requests to appear on Capitol Hill. This week, Sebelius continued a scheduled tour of 10 U.S. cities that included stops at places like Los Barrios Unidos Community Clinic in Dallas, intending to build a “grassroots” coalition for enrollment in health care plans via the online exchange.

“As the administration continues to withhold important details and enrollment figures, I hope Secretary Sebelius is ready to give answers and finally live up to the president’s celebrated claims of transparency,” committee chairman Rep. Fred Upton, a Republican from Michigan, said in a press release.

President Barack Obama, who had promised the “most transparent administration in history,” has been roundly criticized this month by conservatives — and liberals, too — for failing to either “come clean” or “get mad,” depending upon party affiliation.

Whereas state-based exchanges in Vermont, Kentucky, and Minnesota, among others, had enrolled more than 46,000 beneficiaries by Tuesday, the federal healthcare exchange had enrolled far fewer — and was certainly running behind the White House’s goal to enroll seven million beneficiaries by the end of the year. Without exact numbers of beneficiary enrollments, the administration says 500,000 Americans “created accounts” on the website, including Sebelius — who created an account without enrolling, saying she already had insurance.

By way of explanation for the debacle, Sebelius on Tuesday night told CNN that the administration had been given no warning by technology contractors that the site might experience problems after going live Oct. 1. Sebelius attributed problems with the website to “extremely high” volume, though heavy volume might be expected given a stated goal of seven million enrollees. White House Press Secretary said on Wednesday that although the White House knew there’d be glitches, “there is no question that we did not anticipate the scale of problems with the website.”

In response, Republican National Committee Chairman Reince Priebus said in a statement Wednesday, “Either Secretary Sebelius is lying to protect President Obama or the President needs to get control of his signature health care law.”

Technology contractors working on the project this week told the Associated Press, under condition of anonymity, about previous concerns as to whether the product would be ready for launch by the deadline. “They complained openly to each other about what they considered tight and unrealistic deadlines,” the Associated Press reports. “One was nearly brought to tears over the stress of finishing on time, one developer said. Website builders saw red flags for months.”

The administration paid CGI Federal $90 million to develop the website, which still doesn’t work.