SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I have a small lawn with sprayheads. The system worked fine for a long time (2 1/2 yrs). Recently, the spray heads stopped popping up during the scheduled run time. I found a manual gate valve for the sprinklers, and shut it off during the scheduled run time. I waited about 1 minute, then opened it back up. I could hear the release in pressure as water ran down the system to the spray heads, and with that sudden rush of water, the spray heads popped up and operated perfectly normal. At this point I can reliably operate that zone by shutting the gate valve for a short period, and then open it back up. Obviously, this is not a practical way to water the lawn. Any thoughts or suggestions as to what might be causing the problem? The irrigation system also has one other zone running drip emitters. That zone operates normally. I'm very new to this, so any help would be much appreciated. Thanks.