Are you cutting a slot just as narrow as the endmill? Don’t, the high tooling engagement is problematic.
Are you leaving a roughing clearance and taking a full-depth finishing pass? If not, do so.
Have you cut a calibration blank yet?
and calibrated for belt stretch?
https://docs.carbide3d.com/shapeoko-faq/how-to-calibrate-the-machine-for-belt-stretch/