One of the common problems that I've read thru the years on this forum is that the air solenoid valve gets stuck due to rust and corrosion at the needle valve. If it's not used for a long period they will stick. You could try rapping (tapping) the solenoid on the side with a wrench to see if you can break it loose. If you know you're gettting power to solenoid, and tapping doesn't work, then remove the solenoid and clean the gunk out.
Bill (Dune'r) & Helen
Last of the Chieftains, 2004 39T, W22, Banks Headers, Koni's, Safe-T-Plus, SMI brake
2013 Lincoln MKZ Hybrid towed 4-down