I am also wondering if it could be the ride height sensors. If they are not sensing the height of the vehicle and sending the proper signal to activate the compressor? I found this website with some helpful diagrams and info:
BMW Workshop Manuals > X Series E70 X5 4.8i (N62TU) OFFRD > 2 Repair Instructions > 37 Integrated Suspension Systems > 0 General > 7 RA Overview Of Ride-height Control System
This site seems to be full of useful information for all different makes and models. I hope it helps someone as this post helped me so far.