Air vent\duct not enough air is coming
 

My tenant recently complained about one of the bedroom upstairs being
too hot. When I checked the vents in the floor (there are 2 vents) in
this bedroom, one of them has more air blowing than the other one. I
also checked all the rooms, and there is more air blowing on all of
them. Except this one vent in the bedroom does not have enough air
blowing. For this vent that has a problem, I opened the vent and
checked if there is anything stuck which I did not see and everything
seems to ok as far as I know. Is there something I can do to fix this
problem?


Thank you very much for your help.

Annur