Autosampler droppnig vials

I have a G1513A autosampler and the arm keeps either dropping vials or being unable to pick them up.  If I switch out the tray and armsthe problem goes away, so the problem is not the autosampler itself.  The arm drops or can't pick up a vial very frequently and seems to be related to location.  If I keep track of which locations the arm has dropped a vial at and don't put vials there anymore it seems to be ok for a while.  However, I'm starting to run out of locations. The belt in the arm has been changed recently and did not fix the problem. Any advice?