My van had poor airflow from the vents which was caused by three problems.

First the on off switch had disconnected from the flap on one side
which you can see without removing any parts,  secondly the supply
hose had become disconnected on one side,  I also adjusted all of the
control cables and figured out (more or less) the most effective
settings which can change the air flow to the dash front vents
slightly.

I now have great airflow and can keep the windows up on the highway on
warm days.