LOS ANGELES—The Port of Los Angeles is beginning the hard work of convincing terminal operators, importers, warehouses owners, and trucking firms to embrace moving more cargo at night. “It’s not a single lever we can pull today to open up all the gates,” Executive Director Gene Seroka said on Thursday. On Wednesday, the White House gathered stakeholders including retailers Walmart and Home Depot, logistics firms United Parcel Service and FedEx, and electronics supplier Samsung. The companies pledged to use more overnight workers to ease the backups at the Los Angeles and Long Beach port complex that is the No. 1 gateway for trade with China. We’re “trying to squeeze every minute, every hour of efficiency out of this port that we can,” said Seroka. Most work is currently done during the day, although some parts of the operation already run 24/7. The White House said adding more overnight shifts could …