Has it been this way since you got the car, something that may have always been this way but you just noticed, or a new development?
Does the air direction knob move freely? If you switch between floor and defrost, is there a corresponding air flow increase/decrease in those places?
If both of those are true, then perhaps there is something in the duct, leaves, mouse nest, etc. Or, perhaps those ducts got left disconnected when the dash was apart, perhaps for an airbag recall.
If the knob doesn't turn, then there may be something foreign object in the way inside the air box.
If moving the know has no effect on the relative air flow between any of the options, then maybe that control cable got disconnected from either the back of the control knob or the flapper in the air box