The first crewed flight for Boeing’s CST-100 Starliner will have to wait a little bit longer but is still targeting a May launch. The mission is now slated for no earlier than May 21 at 4:43 p.m, NASA revealed Tuesday.

NASA astronauts Butch Wilmore and Suni Williams were sitting in the capsule last week atop a United Launch Alliance Atlas V rocket at Cape Canaveral Space Force Station’s Space Launch Complex 41 with just two hours to go before launch.

Issues with a valve on the ULA rocket forced a scrub of that launch attempt and a rollback of the rocket from the pad to change the valve, pushing the launch date to what was supposed to be as early as this Friday.

NASA said the valve issue on the ULA rocket’s upper Centaur stage was resolved, but Boeing teams found a new issue on the Starliner spacecraft itself.

“Starliner teams are working to resolve a small helium leak detected in the spacecraft’s service module traced to a flange on a single reaction control system thruster,” according to a NASA press release. “Helium is used in spacecraft thruster systems to allow the thrusters to fire and is not combustible or toxic.”

Now teams with NASA and Boeing are working through testing and operational solutions including bringing up Starliner’s propulsion system to flight pressure levels similar to how it would before launch so that the helium can vent and teams can measure it against data to see if it’s working as expected.

NASA said the launch teams were working on no other issues.

The Atlas V and Starliner remain at ULA’s Vertical Integration Facility adjacent to the launch pad while astronauts Wilmore and Williams flew back to Houston over the weekend. The remain in quarantine and will fly back a few days before launch.

The recent delays are small compared with the Starliner program, which is about four years behind SpaceX’s Crew Dragon. Both companies were awarded contracts under NASA’s Commercial Crew Program to provide regular ferry service from the U.S. to and from the International Space Station.

SpaceX completed its crewed flight test in 2020 and has since flown its fleet of Crew Dragon spacecraft 13 times carrying 50 humans into space.

Boeing’s initial uncrewed test flight in 2019 was not able to rendezvous with the ISS forcing more than two years of delay before it tried again. While that trip in 2022 was successful, more issues after the flight and needed fixes to spacecraft hardware have added another two years.

The Crew Flight Test now aims to bring the pair of NASA astronauts for about an eight-day stay on board the ISS testing out manual features on the spacecraft during docking, and then again on its return home to Earth where it will land in the desert in the western United States.