1997 Oldsmobile Bravada Question: Why do I have no air blowing thru dash vents when AC is running.

I have air blowing out at the vent by the gas pedal and out of the window defroster vent on dash top. But none at all across the front dash vents. AC and heater seem to work fine, just no air blowing from dash vents that usually blow directly on driver or passenger.