ac vent issues center vent not blowing
 

i am having issues with the double sided vent in center,both laft vent and right vent have a rolling controll to reduce flow and allow air flow,well the cent vent does not blow air,i move the knob all the way to the top to blow wide open and nothing,id there a vacum control for the vent shut offs of maybe the mechanical linkage is broken,im gonna remove the vent cover and check it out.