Check pressure at the solenoid. Then start closing off the heads one at a time. Sometimes it takes a while for an underground leak to show up. If you close all the heads in that zone and turn on the solenoid you might get enough pressure to force the water to the surface more quickly. Might also attach a pressure gauge at the solenoid to see if the zone supply will hold pressure with the heads off. Then check at one of the heads with the others turned off to see if the zone itself is holding pressure.