I moved into a new home a year ago and have been trying to fix this sprinkler system since. I have 9 zones total and 8 of them work fine. I have one zone with eight 4" pop ups that doesn't have enough pressure to push all the heads up. I was losing a lot of water at startup so i bought the RB1800's with the check valve. Now, 7 of the heads pop up fully but one never pops up at all. I know 8 heads sounds like too many but i have a pressure pump that keeps pressure at 50psi. One of my zones has 7 pop ups running no problem and psi on pump reads a hair under 50psi. I have another zone pushing 6 rotary RB5000's without issue and pump reads 50 psi. However, when u run the zone in question, the pump stays around 40psi. This has to be a leak right? I have dug up each head and traced back to main pipe to check for leaks but found nothing. I dug up all around the valve box to look for leaks there but found nothing. I let the system run for 45 min in hopes of seeing a puddle somewhere but found nothing. Any thoughts?