Is the problem head located much lower than the other heads? It could be that after operating the zone, the problem head is draining the zone line. When the zones active, all the piping is full of water, when the zone shuts off, it could take minutes or hours for the water in the pipe to drain - at the lowest point. If the zone valve is the problem, shut that valves flow control, (top center handle), off then wait a few days to see if the water stops. You may be able to listen to each valve to determine if water is running through them when their supposed to be off. If you find the valves the problem, you'll need to disassemble it and check the solenoid, diaphram, valve seat and orficies for any blockages, rips or tears. But, if water draining from the pipe is the problem, you can purchase and install a check-valve-in head. These can help stop water back flow up to 8 ft. in elevation. Often, this may require additional check-valve-in heads because other may become the lowest point to drain. This is not uncommon on steep lots or where driveways slope to meet the street, or there are elevated planters that water with ground level sprinklers. The water will syphon to the lowest point.....hope this helped