Can not change Heater Air low when switch is turned.

Can not change Heater Air low when switch is turned. I've a 2001 Ford Ranger XLT. When I change the air flow from the heater (i.e. defrost to floor heat). It doesn't respond when the dial is turned. Sometimes after driving a while it will come on to the desired position (After being bounced around). Is there a Manual Cable or hinge that can be lube or is it done electrically?