The upper pendulum roller might be not leveled properly. When regrooving the upper disc for example 0.50 mm in diameter, the height of the pendulum rollerhead must be reduced 0.25 mm. This can be done by the M10 screw on the back side.
See also in the manual X1 how to reset the upper pendulum rollerhead.
NOTE: The description manual is based on a CM X1 welder, but it works very similar for our other welders.
ATTENTION: Do not touch the red marked screws!