If you push your mirror fully forward you'll see a small hole in the top of the mounting - inside this is the long allen screw that tightens the mount onto the door. If you're lucky you might be able to re-tighten it, or undo it to be able to remove the mirror and see what the problem is. I wasn't lucky..
FYI (on mine at least) the allen screw/bolt is a self drilling/tapping item that goes into a captive cylindrical aluminium 'nut' (a bit like an MFI fitting if you know what mean... and that's what pulls everything tight onto the door - however, if the bolt/screw is corroded into the aluminium it is a bast@rd to get loose as there's quite a lot of thread area for the inevitable corrosion to grip the fixing.
Hope your luck is better than mine!
Greg