Health-care workers on the front lines of the coronavirus pandemic are encountering shortages of masks, gowns, face shields and gloves — a frustrating recurrence of a struggle that haunted the first months of the crisis. Nurses say they are reusing N95 masks for days and even weeks at a time. Doctors say they can’t reopen offices because they lack personal protective equipment. State officials say they have scoured U.S. and international suppliers for PPE and struggle to get orders filled. Experts worry the problem could worsen as coronavirus infections climb, straining medical systems. “A lot people thought once the alarm was sounded back in March surely the federal government would fix this, but that hasn’t happened,” said Deborah Burger, a California nurse and president of National Nurses United, a union representing registered nurses. Like many health-care workers, Burger blamed the Trump administration for the lack of equipment, noting the administration has insisted the responsibility falls to state and local officials, with the federal government playing only a supporting role.