If you have a solenoid that stays on with all of the other stations then 9 times out of 10 it is your diaphragm that is playing up. First check that you haven’t accidentally placed it in the ‘on’ position, then check that the small screw for flushing is done up tight. If these are all ok and there are no visible leaks then its usually the diaphragm.
This is the rubber piece in the middle of the solenoid and allows water to flow or blocks its flow. For some solenoids eg Richdel you can probably head down to your local retic shop, grab a new diaphragm and change it over. Other solenoids aren’t quite so easy and you may need to replace the whole unit.
If you are buying a solenoid then I recommend the jar top variety which you can screw on and off rather than the solenoids where stainless screws are used to join the two halves together. In my experience the solenoids that are screwed together are harder to separate and rejoin. Bits of dirt can get in between the surfaces, the gasket can break and it can just be messier getting it all to line up again.