The nation's largest hospital system has made only halting progress in hiring new doctors, replacing incompetent supervisors, upgrading outdated computers and rebuilding trust with veterans, nine months after President Obama concluded that a "corrosive culture" had led to systemic problems at hospitals run by the Department of Veterans Affairs. Now, patients, veterans groups and doctors say delays in receiving care are still common, and they accuse department officials of failing to provide opportunities to see private doctors. Critics, including Republican lawmakers on Capitol Hill, say far too few senior managers have been held accountable for mismanagement at the hospital in Phoenix and at others around the country.