I have a 2005 F-150 4.6L The A/C is working fine with the exception of low pressure coming out from only the 2 drivers side vents. What would cause this?

 

Those 2 vents I keep open at all times. I keep the other 2 closed to get more air flow.

 

The reduction in air flow from those 2 vents just started within the last week or so. I drive a 2005 f-150 for work also, and the the air flow is the same out of all 4 vents. I cant figure out what would be causing these 2 vents to have half the air flow all of a sudden.