2003 Trailblazer why would the AC and heater blow air through the defroster or the floor vents but not through the dashboard vents?

Air door actuator stuck probably. Very hard to replace I've heard

2003+ supposedly have redesigned electric blend door actuators as 2002 was very problematic in this area. Just for giggles, I'd disconnect the batter for 30min to 'reset the computers' to see if that does anything. This has corrected this situation for some.

Joel