Boeing’s BA.N Starliner spacecraft won’t return its two astronauts from the International Space Station until after engineers spend “a couple weeks” on testing to investigate the spacecraft’s thruster issues, a NASA official said on Friday.

Starliner, which has been docked to the ISS since June 6 after ferrying its first crew of astronauts there, has had its test mission drawn out after a series of issues involving its thrusters and leaks of helium, which is used to pressurize the thrusters.

A joint NASA-Boeing team focusing on the thruster issues have planned ground tests at the White Sands Missile Range in New Mexico to test fire the same kind of Starliner thrusters currently in space.

“This will be the real opportunity to examine the thruster, just like we’ve had in space, on the ground for detailed inspection,” NASA commercial crew chief Steve Stich told reporters during a news conference.