a/c compressor not clutching in

95 GMC suburban, replaced the compressor coil, a/c relay and still the compressor will not clutch in. the a/c control switch on the dash "appears" to be working: the light comes on when depressed. the blower motor works fine. this vehicle has 2 a/c systems, one in the rear also. any ideas why the compressor is not clutching in when the a/c is switched on?