It's a possibility that you normally have two valves running at the same time. 3 sprinklers on each valve.
One of the valves might have a problem.
There's a possibility that the line itself is clogged up. Seen it happen. A sprinkler breaks off and the line gets full of dirt. The pipe breaks, the line is repaired but the line is full of dirt. I even found a plastic baggie inside a pipe once keeping half the sprinklers from working.
Could be a check valve in the line not operating anymore for whatever reason.
Let us know how it goes.
Good luck!
This post has been edited 1 times, last edit by "mrfixit" (Mar 21st 2010, 11:52pm)