1. Are fragrance bottles full?
2. Do hoses reach bottom of Fragrance container.
3. Turn unit on, Choose fragrance and hold finger over air on gun to force air backward. It should bubble out of the chosen fragrance container. If not the hose is messed up or the solenoid is not opening. If it doesn't pull hose from hose side of solenoid and see if air comes out. Take hose from container of that unit, disconnect compressor hose and hook up that hose to compressor. See if compressor blows thru to gun.
4. If there are issues with solenoids opening check electrical suppply. If good take apart solenoid and clean. If still not working you may have to replace solenoid valve. Never had it happen on these units.
HAd one goofy item where small poly hose got behind Fragrance bottle sand rested against heater and melted. If solenoids are all good, replace hose assemply and gun.