I've learned that when my rotary cutter does not cut properly it is usually either that I am not holding it straight--the blade should be perpendicular to the fabric, and it's so easy to get it slanted instead--or I am pressing too hard on the ruler instead of the cutter, trying to make the ruler not slip. I'm doing better since I got the Martelli cutter and an Omnigrip ruler. The suction-cup gripper (from Harbor Freight) for rulers has helped also.