cap off one head @ a time to see if there are any leaks that show themselves ,if thats not it you might have a valve problem like a rock in it ,remove top and check after you shut off main .
Vincent Nestor
Nestor's Sprinklers & Lighting
Alpharetta,Ga.30022
vpn1@bellsouth.net
www.NestorsLandScape.com
www.GeorgiaLighting.net