Possible Causes

WARNING: This page is about a different car, the 2002 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.

No vacuum to function selector switch.
Function selector switch leaks vacuum.
Air distribution door binding/stuck.
Vacuum line kinked/pinched.
Vacuum control motor.
Vacuum reservoir tank.
Vacuum actuator arm not connected to door lever.