^ Maybe this.
I had issues with PAC and an ED member told me:
"The EAC and it's dependencies are essential for PAC to work. You will notice the nose move a bit down when engaging PAC mid-air. If any of the EAC's dependencies is gone, apart from the warning indication you will notice PAC, EAC will deactivate and the nose lift a bit. Also opening the air refueling door, disengages both EAC and PAC... unfortunately that is one of the most inconvenient moments to lift the nose."
Basically, make sure the Autopilot system is armed or else PAC will not function.