A pilot program offering 24-hour container operations at the Port of Long Beach hasn’t attracted any truckers more than two weeks since the extended hours began, highlighting challenges facing Southern California seaports as dozens of ships back up off the coast.

Total Terminals International LLC, among the largest terminals at Long Beach, began Sept. 13 to allow truckers to pick up containers between 3 and 7 a.m. Monday to Thursday.

“So...

A pilot program offering 24-hour container operations at the Port of Long Beach hasn’t attracted any truckers more than two weeks since the extended hours began, highlighting challenges facing Southern California seaports as dozens of ships back up off the coast.

Total Terminals International LLC, among the largest terminals at Long Beach, began Sept. 13 to allow truckers to pick up containers between 3 and 7 a.m. Monday to Thursday.

“So far we have had zero deliveries,” TTI Chief Executive Bill Peratt said.

Mr. Peratt said TTI is talking to local trucking companies to see if the terminal can attract drivers to pick up and drop off boxes. ”We want to gather information, make adjustments and see if we can change behaviors,” he said.

The extended hours are among efforts at the neighboring Los Angeles and Long Beach ports to cope with a flood of imports this year that has swamped the critical U.S. trade gateway and snarled inland supply chains. The backups have led to a record number of container ships idling offshore for up to three weeks for berth space, an armada that peaked at 73 ships on Sept. 19 and counted 64 vessels on Wednesday, according to the Marine Exchange of Southern California.

The bottleneck is part of the broad congestion tying up global supply chains as pandemic-driven slowdowns at ports and a rush by Western retailers and manufacturers to replenish inventories overruns freight distribution networks.

The congestion at the California ports, which together handle more than a third of U.S. seaborne imports, is exacerbating soaring costs and shipping delays for U.S. manufacturers and retailers. Containers at the ports waited, on average, 5.4 days to be picked up from the docks by truck in August, according to the Pacific Merchant Shipping Association, a trade group, up from an average 2.5 days before the pandemic.

Most terminals are closed at least several hours a day during the week, with more limited operations on Saturdays and Sundays. The shift to 24-hour operations at one of the Long Beach port’s six terminals was encouraged by the Biden administration as part of its goal of reducing bottlenecks across the supply chain, including at inland railroad terminals and warehouses.

The Port of Los Angeles is maintaining its existing hours. Executive Director Gene Seroka said trucking and warehouse operations so far don’t match up with 24-hour operations and that his port is focused instead on eliminating slack in current operations.

Mario Cordero, the executive director of the Port of Long Beach and a proponent of 24-hour operations, said the port has asked major retailers to direct truckers to use overnight gates, but it will take time to set up new work patterns. “We need their buy-in so they can direct their drayage companies to move this cargo,” he said.

Jonathan Gold, vice president for supply chain at the National Retail Federation, said retailers have concerns about the program’s restrictions as well as a nationwide shortage of the truck chassis that are used to transport sea containers.

“‘If you can’t push cargo out the gate, you can’t pull it off the vessel.’”

— Total Terminals International CEO Bill Peratt

Drayage companies, which specialize in hauling cargo by truck from ports to local yards and distribution facilities, say the overnight system is burdensome. Truckers can only make an appointment to pick up a container if they are able to drop off a specific type of container and chassis during the same run. Getting everything to match up can be difficult, said Matt Schrap, chief executive of the Harbor Trucking Association.

”I commend TTI for what they are trying to do,” said Mr. Schrap, who represents hundreds of West Coast motor carriers. ”But if you can’t get the appointment in the first place because it doesn’t line up, then it doesn’t make a difference.”

TTI’s Mr. Peratt noted the terminal accepts containers from seven ocean carriers but that truckers are limited by the types of chassis they are able to drop off.

He said the terminal is exploring ways to make the program more productive because being able to move more containers off the docks allows the terminal to unload ships more quickly. “If you can’t push cargo out the gate, you can’t pull it off the vessel,” he said.

Write to Paul Berger at paul.berger@wsj.com