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The first thing I would recommend is checking the cabin filter. The cabin filter is in the heater box behind the glovebox opening, air in the heater box flows through the cabin filter before reaching the vents. If the cabin filter is plugged then it will prevent airflow.
If the cabin filter is fine then you may have a restricted outside air intake, foreign debris in the heater box or a door issue. Try running the system on recirculation if you haven't already and see if that does give you airflow, if it does then you know the outside air intake is restricted.
It's possible that you could have a broken or bound up door inside the heater box that is preventing airflow also.