Like air brakes, bags can be affected by water in the tank(s). If there is a petcock(s?) to drain the system somewhere at the lowest point of the system try releasing all air and see if some water shoots out. If the compressor dryer is not working then that can cause water problems. If your system is watering up then you might be able to fix things by simply drying out the lines and tanks. Something truckers do if they know what can happen to air systems if not checked for water in the lines frequently.
I have lost air brakes in the past because of dryer problems during cold and damp weather on a Mack hauling 30 tonne of pit run simply because the tank watered up and froze the lines sticking the brakes off. Not a fun ride, but I did have a spare pair of shorts back in my locker so the lesson was not lost on me