1) Material Inconsistency
Most materials have inconsistencies across a single piece. Knots in wood, holes/gaps in the material, changes in density, and patches of glue are just some of the examples that could cause this. Laser Not Cutting Through Material Troubleshooting.
2) Mirror Alignment
Perform the 4 corner test to ensure that your mirrors are properly aligned. If not, then align your mirrors to improve laser output.
3) Nozzle Alignment
If your nozzle cone is not aligned, your laser beam could be clipping the side. Double check that you can see the diode shining through the cone and that it’s not loose when you touch it. If needed, adjust and retighten the screw.