Possible causes:
- a mechanical difficulty — check pulley set screws, Z-axis belt tension (if using a belt drive)
- an electronics issue — check the wiring and connectors
- the endmill gradually working loose — check that an adequate length of shaft is in the collet and tighten using two wrenches per the instructions
- the endmill getting pulled into the cut — keep tooling engagement as low as possible — esp. avoid slotting, where appropriate add geometry and cut as a pocket down to at least tab depth: Adding geometry to cut as a pocket with a finishing pass