Test pulley set screws, making sure they are tightened all they way
Re-tighten if loose. If this occurs often, consider using thread locker (such as Loctite) to prevent the set screws from coming un-done.
Test belt tension, it should be taught but not overly so
Follow the belt assembly instructions to reset the belt
Reposition Z-Axis stepper motor to adjust belt tension
- Depth is wrong