It could be the mat. I had the same thing happen. I had bought a new cutter at the quilt show and extra blades but it wouldn't cut, it skipped. I couldn't figure out the problem. Then I tried something, I got a different mat out and it cut great. The mat I was using was new, it was one of the large ones from Joann's that fit on top of my cutting table. well it wasn't self healing because the only place I could cut on that mat was a place that I hadn't cut on before. That was the biggest waste of 50 dollars that I'd done in a long time. Try it and see if a different mat makes a difference.