WASHINGTON — The sudden delay of a major part of President Barack Obama’s historic health care overhaul is raising questions about other potential problems lurking in the homestretch. The requirement that many employers provide coverage is just one part of a complex law. But its one-year postponement has taken administration allies and adversaries alike by surprise.White House officials said Wednesday that the delay was firm and won’t be extended after a year — and that the overhaul will still be fully implemented by the time Obama leaves office. But the officials, who were not authorized to discuss internal deliberations on the record and spoke only on condition of anonymity, wouldn’t rule out delays or tweaks to other provisions. The White House action means that some companies that would have offered health insurance next year to avoid fines will not do so now. They’re mainly firms with many low-wage workers, such as restaurants, hotels and temporary staffing companies. The workers, however, will still be able to get coverage. Many may qualify for subsidized insurance through new marketplaces to debut Oct. 1, less than three months away. The fact that new problems are popping up at this late stage could be a sign of additional troublesome issues ahead. It underscores a recent warning by the Government Accountability Office that the “timely and smooth” rollout of the new insurance markets can’t be guaranteed, partly because much of the technology to run them hasn’t been fully tested.