nitrous problem

Do the tests TT5.9mag suggested.
Both times this happened to me it ended up being the nitrous solenoid itself. My system would stay on the car while it sat over the winter, then it would not work with same symptoms the next summer. Solenoid seamed to bench test ok but replacing it fixed the problem, like it only failed to open while engine is running.
Now when my car sits for the winter I remove the plate and solenoids and store them in the house and problem never happened again.