Just keep tightening the screw. As the screw tightens, it will draw down the mirror stem into the mounting hole. It is almost impossible to get the mirror stem to seat by hand. At least that was my experience. I put in some Never-Seize on the stem.
EDIT: They will still be able to move a bit laterally if you push/pull on them, but should not move while riding. If they move, just put in a flat washer under the wavy washer and tighten it up a bit.
H.
Last edited by hyates; 03/31/2011 7:09 PM.