A bid by SpaceX to carry out the first U.S. commercial resupply mission to the International Space Station (ISS) is unlikely to lift off May 7, the latest target launch date, according to the company.

“SpaceX is continuing to work through the software assurance process with NASA,” according to a May 2 update from company spokeswoman Kirsten Brost Grantham. Earlier, the company slipped a planned April 30 liftoff to allow more time to work software flight-control issues for the planned 18-day test mission.

Launch opportunities for the Falcon 9 booster with the unpiloted Dragon supply craft fall once every three days, opening the possibility of a May 10 liftoff from Cape Canaveral.

Though the NASA-led ISS mission management team is not setting the company’s launch dates, it is controlling access to the station once the SpaceX Falcon 9-launched Dragon supply ship reaches orbit.

Upcoming restrictions include Russia’s planned May 15-17 launch and docking of the Soyuz 30S spacecraft with two cosmonauts and a NASA astronaut. The newcomers will return the station to six-person operations, following the return to Earth of three U.S. and Russian station astronauts on April 27.

With each slip, the orbiting science lab is also closing in on a restrictive period of high solar beta angles during late May and early June.

SpaceX conducted a mission countdown rehearsal and first-stage hot-fire test on April 30.

As the test concluded, SpaceX was aiming for a liftoff on May 7 at 9:38 a.m., EDT. The mission is intended to demonstrate the automated supply ship’s ability to rendezvous safely with the station.

On the mission’s third day, two astronauts on board will be prepared to grapple the freighter with Canadarm2, the station’s 58-ft.-long robot arm. If successful, they will berth Dragon to the Harmony node on the U.S. segment for a 15-day stay.