on mine.... there's a little set-screw on each side of the arm that holds the eye. These had come loose, so it would wobble all over and miss the mark. Snugged them up, and all good.
A second problem (still to this day, 3 years later...) I've calibrated it - yet I always have to start it out a good 1/8 inch below the actual mark, instead of right on it or it just won't find it. This happened about a year after I got it. Now I "know" so I just allow for it every time, and it works perfect.