NASA delayed the launch of space shuttle Discovery for a fourth time on Friday amid valve concerns, and managers are uncertain when the flight will take place.

Following a 13-hour meeting at Kennedy Space Center, shuttle managers decided against launching this month. The launch had been targeted for no sooner than Feb. 27; no new date was set.

Officials said they believe they have a realistic shot at launching Discovery to the international space station before mid-March.

After that, the shuttle would have to get in line behind a Russian Soyuz launch with a new space-station crew, and the next opportunity for Discovery would be after April 6.

• Click here to visit FOXNews.com's Space Center.

NASA originally hoped to send Discovery to the space station, with one final set of solar wings, on Feb. 12.

But extra tests were ordered for the valves that control the flow of hydrogen gas into the external fuel tank during liftoff. One of those valves broke during the last shuttle launch in November.

Shuttle program manager John Shannon said "we were really, really close" to approving the launch. But some of the test data were not available until late in the week and did not allow for full scrutiny, and a few small mistakes were found in the data.

"There was just a sense of unease that we did not quite have the rigor that we typically expect for a question like this," Shannon told reporters late Friday night. "It's a very complicated problem."

He said more tests will likely be needed.

NASA wants to make sure if another valve breaks, Discovery and its seven-man crew will be safe. Officials said they're still not certain why the valve cracked on the last flight.

The three valves are located in the shuttle engine compartment and route gaseous hydrogen into the fuel tank to keep it pressurized properly.

Only nine space-shuttle flights remain.