HELP! sprinkler system

I have installed a sprinkler system in my backyard and ran into some problems. I hooked up an anti-siphon valve in the front yard that goes to three valves. Two of the valves go to the backyard for the sprinkler system. I have them hooked up to a Rainbird electrical controller. The problem I have is with the second valve that is set up for the east portion of the sprinkler heads. With the second valve I have 11 sprinkler heads and with the third valve I have 10 sprinkler heads setup, which is on the west side of the backyard. Anyway, all was well for the first week and then my east set of sprinkler heads, which is on the second valve, only goes to about half of its "flow potential", it's as if those sprinkler heads are now not getting enough pressure for them to run at their greatest potential. Now the east side of the backyard get flooded near those heads because the water just shoots out about two feet. I checked the second valve, valve in question, and opened it up to see if there was any objects blocking the valve. The valve looked clean and good-to-go. I tested the sprinkler system again and the same problem occurred. I have pop-up sprinkler heads that do not go all the way up on this second valve. I decided, while they were running, to pull the pop-up head on one of the sprinklers connected to the second valve and they all then popped up and seem to work correctly. What is going on? Can anyone help me on this issue?
Thanks, prs1972
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