Please check the machine mechanically:
- Pulley set screws: http://docs.carbide3d.com/shapeoko-faq/shapeoko-3-how-to-check-the-pulley-set-screws/ — be sure to check all axes/pulleys including Z.
- V wheels / eccentric nuts: http://docs.carbide3d.com/support/tensioning-eccentrics
- Belt tension — the Z-axis should be guitar string tight (but careful not to bend the motor shaft): http://docs.carbide3d.com/assembly/shapeoko/xxl/step-5-belting/ on deep cuts it may help to remove one spring from the Z-axis temporarily, esp. if one hasn’t added a spoilboard on top of the wasteboard — it also helps to install the router as low as possible (installing the Makita adapter upside down will help). Some folks have found it helps to remove bolts which won’t stay tensioned (M4 Z-axis tension bolt, various V wheels with eccentric nuts), apply a thin bead of threadlock along the length of the threads, then reinstalling. See the video at: https://www.youtube.com/watch?v=_lIIb_PdziA Note that the X-axis motor is held in place on standoffs and if those bolts are loose this can cause belt tension issues.
It is also important to be sure that the collet is correctly tightened, the endmill fits correctly and doesn’t slip, and the router is mounted securely in the mount, and that the mount doesn’t shift. Note than endmill pullout can happen gradually, especially when profiling against tall walls.[4]
Also feeds and speeds may be a consideration: https://docs.carbide3d.com/support/#tooling-support and see https://www.youtube.com/watch?v=S9bceJxpqG0 for concepts on this and https://www.precisebits.com/tutorials/calibrating_feeds_n_speeds.htm for a testing technique and see the series #MaterialMonday: #MaterialMonday on YouTube
Beyond that it’s usually a matter of ​Calibration and Squaring the Machine c.f., http://docs.carbide3d.com/shapeoko-faq/how-to-calibrate-the-machine-for-belt-stretch/