A/C clutch not engaging (2000 Subaru Legacy)

So far I've ruled out the relay, fuse, and too big air gap.  (I actually removed
both shims and tightened down until the clutch spun regardless of whether
the A/C switch was on or not -- it made a squeaking noise so I turned it off
after a few seconds and reinstalled the shims.)  Checked the pressure at low
pressure port and added refrigerant.  As it came to 'normal pressure' I  
heard the engine 'rev up' as if the A/C was coming on, but the clutch plate
stayed still.  At this point, the engine reacts when the A/C switch is turned
on, but the clutch doesn't engage and the air stays hot.  What next?  Thank
you!!