The first-ever crewed flight test of Boeing’s Starliner space taxi has been rescheduled for no earlier than May 21 to give Boeing, NASA and United Launch Alliance extra time to complete on-the-ground testing.

An initial attempt to send the gumdrop-shaped capsule and two NASA astronauts to the International Space Station had to be scrubbed on May 6 due to concerns about a fluttery pressure regulation valve on the Atlas V rocket’s upper-stage oxygen tank.

Starliner and the rocket were rolled back to the Vertical Integration Facility at Cape Canaveral Space Force Station’s Launch Complex 41 in Florida. The valve was replaced on May 11 and passed a round of tests — but along the way, engineers detected a small helium leak that was traced to a flange on one of the Starliner service module’s thrusters.

NASA and Boeing say that they’re developing spacecraft testing and operational solutions to address the issue. Today’s status update suggested that the flange won’t need to be replaced.

Liftoff from the Cape is now scheduled for 4:43 p.m. ET (1:43 p.m. PT) May 21. NASA astronauts Butch Wilmore and Suni Williams are due to take Starliner on a weeklong shakedown cruise to the International Space Station and back. If everything goes according to plan, Boeing’s Starliner would join SpaceX’s Crew Dragon in the rotation for crew flights to and from the space station.

A series of technical snags has led to years of delays and more than $1 billion in cost overruns for the Starliner program. Under the terms of NASA’s fixed-price contract for Starliner development, Boeing has had to cover the extra expense.