Most of those compressors have a vane type pump with 2 spring loaded carbon (graphite) bars for vanes. I have had these stick in the retracted position (they spin in an offset oval housing, big side with vanes extended for intake, small side with vanes pushed in for compressed air out) so the motor spins the compressor but it doesn't do anything. If they are sticking, you have to clean the vanes and the slot they ride in. Don't oil them or they will soon stick again.
