Boeing is continuing to battle production issues plaguing the jets that will serve as Air Force One, according to the Government Accountability Office (GAO).

The Pentagon told the congressional auditors that “ongoing design issues, modification rework, and workforce challenges slowed the progress toward modifying two Boeing 747-8 aircraft into presidential aircraft,” a report released Wednesday said.

The United States, under President Donald Trump, signed a $3.9 billion contract with Boeing in 2018 to have two planes designed, modified, and tested to serve as Air Force One by 2024, but persistent delays have pushed the project past its delivery date.

In order for the aircraft to meet Air Force One specifications, Boeing needs to heavily modify the jets to meet travel and top-level security needs, which has slowed down the production process considerably. Trump has dropped a requirement for the VC-25B planes to have air-to-air flying capabilities, which would allow them to receive fuel from a tanker aircraft while in flight.

Progress on the aircraft has stalled as a result of issues around decompression and the environmental control system design, and Boeing’s incomplete certification plans and aircraft design have also pushed back the timetable for testing, the GAO said. The aircraft manufacturer has also been unable to keep a steady workforce for the project.