1999 Mitsubishi Eclipse A/C PROBLEM

Air Conditioning problem
1999 Mitsubishi Eclipse 4 cyl Front Wheel Drive Manual

The a/c works fine in defrost mode; the compressor clutch engages & cold air blows out of the defrost vents. When I turn the control knob over to a/c venting the clutch disengages & the air goes warm. There is a button on the dash for air recirculation; my understanding is that that button needs to be pushed in to engage the clutch. With the button in the clutch still doesn't engage.