One of the belt locks isn’t locking properly.
Ensure belts are engaged
Check belts and make sure that the lever on the box is engaged and the belts are aligned with the pulleys properly with adequate tension.
Reset to Factory Defaults
Reset the values for the motor settings to factory defaults. Go to Home > Setup > Advanced > Factory Defaults
The motor box may need to come in for an evaluation. Contact your local dealer or Quilt-EZ for assistance.