The diagram on a valve uses the pressure difference on the front side and back side of the valve, plus the solenoid plunger to actuate. If there is a tear in the diaphragm, or debris that would not allow it to seal, then the valve can’t create the negative pressure seal. I would lean towards the diaphragm being an issue, but it also could be a solenoid. I just had to replace one in my yard. I got home to one of my sprinkler zones running at about half pressure. Found the solenoid to be completely blown out the side, and the plunger was stuck partially engaged. I don’t know if it stuck during a run or not.
A crude drawing to somewhat illustrate.
