A/C Heater air flow

On my 99 montana the air flow doesn't change when the position selector knob is moved to a different position. Most of the air comes out of the floor vents, but a small amount comes out of the upper vents. I have checked the vacuum lines in the engine compartment & they are good. Does anyone have any ideas what My problem could be & how to fix it? Thanks

Go under the dash on the passenger side and find the vac source line....with the car running, does it have vacuum? It is usually purple or black in color....