Possible causes:
- mechanical issues — check the machine mechanically
- wiring fault — usually this is accompanied by awful noises
- feeds and speeds —check that these are suitable to the material/tool
- chip recutting — make sure that dust collection is removing material and it isn’t getting packed into the cut
- high tooling engagement — add geometry and cut as a pocket down to tab height or the penultimate pass, see: Best Bit for full cut through - #2 by WillAdams