Air traffic controller staffing shortages worsened over the weekend as the nation’s government shutdown hit its fourth week, leading to delays and anxiety, and experts say it won’t get better until air traffic controllers get paid.

More than 50 staffing shortages have been reported since Friday morning, causing delays from Los Angeles to Washington, DC, according to an operations update. Controllers are considered essential workers, so they must work during the shutdown, but are not being paid.

Flights for Los Angeles International Airport were temporarily halted Sunday because of a staffing shortage at the Southern California TRACON, which handles flights arriving or departing. At Reagan National Airport in Arlington, Virginia, staffing issues caused a ground delay Sunday that was expected to last until midnight.