No air coming out from one of the two mid high air vent holes in the middle

I switch the airflow to let fresh air come out from the mid high vent holes. The right one is blowing air, but not the left, that is the one to the right of the steering wheel (left drive car).

I have searched and read other threads about bad flowing air, etc., but that seems to be for all vent holes. Why does this only affect one of the holes? I find it hard to believe something could have been clogged. Could the butterfly mechanism for shutting off air be stucked in closed mode for just this vent hole? Or how does these mechanism works when I turn the settings for where the air should flow, down to the feet, to the window, in the middle, etc?

Thanks for any help.
I now found the small wheel under the vent that open and shuts off the vent from blowing air. It was closed. Never seen this during all these years. Well, everyday something new :)