Chevy impala a.c. air

Check the actuator under the dashboard, it may not be opening and closing the vents correctly.

Jorge is right. But, those friggin things are a pain in the butt to get to. Crawl into the floorboard of the passenger side and see if it clicks when someone else changes the dials from defrost to panel. If it doesn't make sound there's the issue. It should barely be noticeable but it will be if you have your head close to the center of the car under the glove box.