The ductwork has blend doors that move to get the temperature to the vent where you want it. Could be that one of the blend doors is sticking because the door actuator is defective. You may have noticed a tapping sound under the dash from a defective actuator. The doors are located under the dash and possibly behind the glove box; the actuators are small boxes. Try a spray lubricant on the actuators, or remove them to see if there's a broken sprocket. Might need replacing. It's not a difficult fix.
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