No AC airflow through rear vents

The AC vents in the upper rear and third row have no airflow. The airflow seems fine in the from and lower rear and it is cooling fine but the other vents mentioned have zero air coming from them. I've searched my user manual and looked online and can't seem to figure out if they aren't working or if I just don't know how to get them to operate but I'm almost positive th eery were working last year when I purchased the vehicle.