Make sure there's nothing under your mat and that you have it on a hard surface. Someone about 2 weeks ago was trying to use it on a carpeted floor.
I prefer my green mat from Joann's over the gray Fiskars from Walmart. I also have a June Tailor Cut & Press that sits by my machine for trimming and it is sort of bumpy so it will cause the blade to skip if I don't put enough pressure on the cutter.