Please check your machine mechanically :
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.
The router speed setting should match the feeds and speeds for your selection of endmill and material. Please see:
(there’s a row matching dial settings to RPMs at the bottom)
There’s an interactive version at: https://public.tableau.com/profile/willadams#!/vizhome/Carbide3DCNCFeedsandSpeeds/Sheet1?publish=yes
See https://www.youtube.com/watch?v=S9bceJxpqG0 for concepts on this and see the series #MaterialMonday: #MaterialMonday on YouTube for specifics.
You should test feeds and speeds in a piece of scrap using the technique at: https://precisebits.com/tutorials/calibrating_feeds_n_speeds.htm
Another consideration is Climb vs. Conventional Milling and tooling engagement — where possible avoid slotting and add geometry and cut as a pocket (Adding geometry to cut as a pocket with a finishing pass ).
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/