Hi I am here to help you honestly and with the utmost integrity.I may ask you questions back to minimize bad information. We need to check process.
When you switch it does it make any sounds? Or is it hard to switch resistance whe your turn it!
think of the vent system as housing ducts there are doors that open and close to for each if the door is not opening then the door or motor for the door is not getting power or malfunctioning. The best way to find out is by taking the dash apart to find what is wrong with the doors. If you have top take it somewhere again most of this is not mechanical work is body labor take it to a auto body shop dealer or a independent that has a mechanic on duty