A while ago I used some special 3M interior glue spray on my driver-side A&B, and passenger side C pillar. Finally gave in because the C pillar fabric was fluttering in my rearview mirror. Two months later, the drivers-side C and passenger B are starting to do it. sigh.
The real issue is that BMW used just enough fabric to cover what you see, not enough to wrap around , and that's why it gets loose.