vents

Small problem with my 2002 Alero. Air will come out of the defrost and floor vents but not out of the vents across the front of the dash. Just wondering if anyone has had this problem or if anyone has any ideas as to how to correct this problem. Thanks to all who reply.
-Eric-

Which engine do you have? Check the vacuum lines on the upper intake manifold and throttle body. There is a accessory vacuum line that goes from the intake manifold to the HVAC system under the dashboard. The actuators and controls are both electrical and vacuum operated.