You just notice the rear of the truck sagging when it does not work?
When the pump is running, everything works OK?
Have you had any previous air suspension work done?
If the compressor, air suspension module, compressor relay are working properly and you have no air leaks, the likely cause is an open between the air suspension module and the air suspension relay. Circuit 420 has a dark blue/ yellow wire. The air suspension module grounds this circuit to energize the relay and turn on the air compressor. I have run into this problem a few times on Expedition and Navigators after the above parts have been replaced. It may be an intermittent concern that I have found hard to duplicate. The guys at the Ford hotline had me overlay circuit 420 from the air suspension module to the air compressor relay. It was a chore because part of the dash had to be removed to access the module. Once I overlayed the circuit to the relay, it never came back for that concern. I was able to help two other customers here on Just Answer with the same concern. I'd get the codes checked first to be on the safe side.