WASHINGTON — President Obama is enjoying a winning streak lately, with the Supreme Court reaffirming his signature health care law and Iran agreeing to curbs on its nuclear program. But one longstanding goal continues to bedevil him: closing the wartime prison at Guantánamo Bay, Cuba.

The administration’s fitful effort to shut down the prison is collapsing again. Ashton B. Carter, in his first six months as defense secretary, has yet to make a decision on any newly proposed deals to transfer individual detainees. His delay, which echoes a pattern last year by his predecessor, Chuck Hagel, is generating mounting concern in the White House and State Department, officials say.

Last week, Mr. Obama’s national security adviser, Susan E. Rice, convened a cabinet-level “principals committee” meeting on how to close the prison before the president leaves office in 18 months. At that meeting, Mr. Carter was presented with an unsigned National Security Council memo stating that he would have 30 days to make decisions on newly proposed transfers, according to several officials familiar with the internal deliberations.

But the meeting ended inconclusively. Mr. Carter did not commit to making a decision on pending transfer proposals by a particular date, including the repatriation of a Mauritanian and a Moroccan. Nor was it clear whether he accepted the 30-day deadline, those officials said.