Boeing’s second uncrewed flight test of its Starliner spacecraft is delayed for at least two months due to issues with the capsule’s propulsion valves, the company said on Friday.

The latest mission – called Orbital Flight Test 2, or OFT-2 – was previously targeting December 2020, but Boeing delayed the launch multiple times, with Aug. 3 the most recent target. During preparations on launch day, Boeing discovered that 13 propulsion system valves were not opening on the spacecraft, causing the company to delay the launch.

While the company’s engineers restored functionality in nine of the 13 valves over the past week and a half, Boeing vice president John Vollmer said the team “exhausted every possible option” to repair Starliner while the capsule was on top of the rocket – necessitating a return to the company’s processing facility for further investigation.

Vollmer said Boeing is working with Aerojet Rocketdyne, which manufactured the valves, to identify the exact cause of the problems and analyze possible preventative measures or redesigns.

The additional work means Boeing will not have an opportunity to launch OFT-2 this month, NASA Commercial Crew program manager Steve Stich told reporters, and is delayed “definitely on the other side” of an agency mission planned for mid-October.

The announcement of the OFT-2 delay comes about 19 months after Boeing’s first flight test went awry.

OFT-2 represents a redo of Boeing’s first uncrewed flight test in December 2019. That first Starliner mission was cut short when, after a successful launch, the spacecraft’s flight control system misfired and the capsule did not make it to the International Space Station as planned. While Boeing was able to conduct tests of many parts of Starliner during the shortened flight, NASA declared the flight test a “close call,” saying that the spacecraft could have been lost twice during the mission.

The company has made dozens of changes after an investigation alongside NASA. Additionally, Boeing is covering the cost of OFT-2, having set aside $410 million shortly after the first flight test. Vollmer on Friday said he was not sure how much the delay and additional work will cost Boeing.