Air won't blow from face vents in rear. Only blows from floor vent

The rear air won't flow (heat or A/C) from the roof vents in the rear of the van.  It blows
from the floor vents from all settings.  It will adjust flow based on setting (1, 2, or 3).  I
have tried adjusting the vent setting from the front controls and from the rear controls.  
Neither will make it blow from the ceiling vents.