1997 Ford F150 No air flow at floor

I'm unable to get heater air flow at the floor vents. The blower works in all speeds. I can select defrost only and panel only and get plenty of air where its supposed to be. When I select panel and floor combined I only get panel - nothing at the floor. When I select defrost and floor combined I only get defrost - nothing at the floor. When I select floor only I only get defrost. I'm able to change temperature from hot to cold - just can't get air out of the floor vents.