"The reasons for this are not clear. Are website failures preventing enrollment? Are consumers balking at the price of coverage?"
ObamaCare's health insurance marketplaces opened for business Oct. 1.
The federal enrollment portal, healthcare.gov, succumbed to traffic and design problems and was inaccessible for most of the week.
Technical problems were beginning to ease by Friday, but users still encountered challenges creating their accounts and comparison shopping for health plans.
Republicans have used these failings to declare the marketplaces unworkable.
They have also blasted the Obama administration for refusing to release initial enrollment figures.
White House press secretary Jay Carney said Monday that federal officials would release the numbers each month.
"What I can confirm right now is that people are signing up through federal exchanges, but … we’re not going to release data on an hourly or daily or weekly basis," Carney said.