irrigation- one zone won't come on

so one zone won't come and it used to. all of the other zones/valves work fine. i tested voltage and resistance at the controller and at the valve (and at the solenoid) and they are all what they should be. if i turn the solenoid on manually a quarter turn, the sprinklers come on like they should. when the controller tells the zone to turn on, i hear water flow but nothing happens, when the power is off to the valve, i hear the water stop and the solenoid clicking to close. what am i missing?