Hi, Wanda -

After the CE replaced the drive, he/she may not have taken the additional step 
of a re-teach in that frame (because it is time-consuming, and CEs want to get 
on to the next call).  The robotics reposition themselves on the basis of 
memorization, based upon reflective "fiducial" spots located in various 
positions in the frames, which they sense and remember during a teach 
operation.  If positioning is just a little off (due to track settling or other 
mechanical shifts over time), irregularities in operation can occur.

    Richard Sims, at Boston University