| I would imagine the center vent flap controls the airflow on the center vents and it's vacuum actuated. The actuator is near the dash surface so it's exposed to heat. My idea is the rubber diaphragm tears after a while not unlike those ones buried deep in the dash.
Believe that on cold air with compressor on (middle button) the center vents should blow air, but on warm/heating mode, they don't. | 
