Look under the dash on the driver side toward the center of the dash. You will see a couple of vacuum actuators mounted to the side of the HVAC case. Down at the bottom of the case, there is a linkage that connects to a white lever. Operate the heater mode control while observing the linkages. You will notice the white lever will wobble around. The lever is broken where it attaches to a door.When this happens, the floor vents will not open. You will have to remove the HVAC case to repair this. The parts cost about 30.00.
Also, make sure you have vacuum to the mode actuator.