I have ran into this problem with asco, burkert, parker, and a few other brands of solenoid valves, and the majority of the time it is due to a small spec of material stuck on the rubber diaphragm. I would start by removing the plastic cap from the top of the solenoid, then the retainer clip below it, which will allow you to remove the solenoid coil. Then, remove the screws holding the valve body together, remove the rubber diaphragm and clean it with rubbing alcohol. The majority of the time you will be able to see the offending material on the diaphragm, although, I have cleaned a few without any visible issues that have been good to go afterwards. If that still doesn't work, then I would start looking into a rebuild kit or new solenoid coil if needed. If you want to eliminate the solenoid being bad, just swap the solenoid from the other valve and test it.