To help avoid confusion, there is only one blend door, which allows air to fully pass through the heater core, or to fully bypass the heater core, or anywhere in between which would be "blending" the air flow to the selected vent.
The rest of the doors are mode doors that control where the air goes (defroster, dash vents, floor, etc) or where the air comes from (fresh air/ recirc air).
I would be looking for something that could be blocking full air flow.
- John