Sounds like it's needs recharged if it drops below certain level the compromise kick on as to not damage the unit when running it out. I would start with getting recharge kit and properly draining the system of all it's air then recharge it as can tells you to do so. Once system is clear of air you can then replace it with your A/C freon until your level is back up in the safe green area. Once it does you see your compressor starting to kick back on and off make sure vehicle is running and A/C is on max when adding the freon