I use an electric sharperner on mine. Some tips will break if the leads are broken inside the pencil. I had one that I could not sharpen but most of the others worked just fine. They do recommend their sharpener but I broke the rules and used my electric with no problems.
I just bought a set of 36. I sharpened them yesterday and used a makeup sharpener. I had trouble with 3 pencils. I emailed the company today and they told me they are sending me the 3 pencils I had trouble with. Then they sent me a pdf form to tell me the correct way of sharpening their pencils. They do recommend to use electric sharpeners the first time you sharpen them. Then to keep them sharpened using a makeup pencil sharpener or theirs. Also you should not drop them or the air plays a role in the wood. Drying out or too much humidity can make them swell.
I am not sure but at one of my classes the instructor mentioned that she would take a Prisma pencil that would not sharpen and put it in the microwave for approx. 10-20 seconds. This melts the color and then it resets. The cracks come from dropping or shipping. I have not tried this yet but if soneone does, please let me know if it works.