You have to test the pressure at the beginning of the line and also at the house connection while the zone is operating. You could have too small a pipe if the pressure at the beginning is still in the 60-80 psi range. If the beginning pressure is also low, but the house pressure is high then the problem is your operating valve or inlet piping. If you connected to a small pipe at the house, then it could also be too small to operate the sprinklers. You may have to have a plumber check the system.