The flaps in vent do not flow air in the right positions when tuning knob
www.youtube.com/watch?v=Ow0OOfIFtlU -- sounds like your mode door actuator is not working or the control panel is faulty. Very common on these trucks. Check out this video for some insight into solutions.
Search Chevrolet Silverado 1500 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Chevrolet Silverado 1500 Experts