It is likely that one of your flap motors is faulty or a vent is stuck. Do a scan with vcds or obdeleven as the motors give a code when faulty. Also look at the live data when operating the flaps, you will see from the open and closed values which one is faulty. If you don't have a scanner, the fault you are describing is normally the V71 Recirc flap motor. The recirc flap motors are next to the fan behind the glove box, you will see two 6mm bolts, undo them and release the tabs at the top and front right, you will need to move the electrical box on the right of the fan (2 10mm nuts), to have enough space to remove it. Move the assembly to the right then pull out. The V71 motor is the one at the back with the white cog.