If manual heater controls, not ECC, the controls operate flaps via short stiff cables. Something may have broken off or need a little teflon lubricant. You can gently pull the control knobs forward, exposing 2 screws that will let you remove the little subpanel and look in. Be careful, it's easy to break the various plastic parts. May have to remove kick panels on one or both sides to see the cables properly.

If you have ECC, the flaps are motor-operated and my comments don't apply.