I have a system with 6 zones, 1 of the six has 4 rain bird 5004's on it. My static pressure is 52psi. Heads are nozzled at 8gpm. Turned on the system this morning and it worked fine. Pressure runs at about 23psi, turned it on again manually to adjust and it won't pop the heads up but only half way, pressure goes down to about 11psi. Any ideas?