'Could be an issue with your mixer doors.... check other threads on this problem.When I put my AC up on defrost, it won't leave defrost, and blow back out the regular vents until I turn the truck off. It won't blow out on feet or all four of the main blowers. It will continue to blow some air out from the outside vents on each side, but not all four.
If you have "dual" climate control, try switching back & forth between common & dual, and between defrost mode, floor mode & A/C mode..... playing around with this 'might' break loose a "stuck door" problem. That has happened to myself & others. The more proper fix of course would be to get in there & fix that malfunctioning mixer door. But doing this "hick-fix" will validate that the door is in fact your problem. (under dash)