My VX has this issue, the tolerrances on the ASA on the dove tail is pretty bad. I actually slipped a shim betwen the frame and ASA and tightened the two screws and viola. No movement what so ever. Kind of sucks that i had to do this but you get an ASA that doesn't move when getting banged around.
if you are having an issue and it is still under warranty you are more than welcome to send it in so we can look at it. If you have purchased the marker used or is out of warranty we can still look at it for you.
What we see alot is that people over tighten the ASA and stretch the Bevel causing it to be loose even after you tighten it. May need to replace the ASA if that is the problem.