If you're experiencing any sort of variation in depth across your material while carving, it may be that your wasteboard isn't completely flat relative to your spindle. The best way to make sure your wasteboard is flat, relative to the spindle, is to use the machine itself to skim off or plane the top layer of your wasteboard. This is a required step for the best carving results, especially with v-bits and fine detailed carves, and we've included this step as part of the assembly.
Over time, with changes in temperature and humidity, you may see minor inconsistencies in your carving depth reappear. The MDF wasteboard material, although manufactured to be flat, can still expand and contract with those temperature and humidity changes. To correct this, simply go back through our planing project to skim off the top layer of your wasteboard one more time. Planing projects can be found here:
This is a technique that can be used on other 3D Carving machines to ensure a flat wasteboard surface and here are a few additional examples showing this from our Community Forum: