if it's anything like my ole mustang u might have a vacuum leak. my stang had el cheapo plastic vacuum lines that were 15 years old and had broken/cracked. the outcome was no vent air only floor and def. my mechanic (while charging the a/c) noticed this and put now lines on and after that u could here the mechanism switching from floor to vent etc.
check it out and see if the bmw system is vacuum actuated.