Is your ruler slipping around while you are cutting? I had this problem and tried the little dots, walking my hand, etc. none worked. I decided to try weighting it down and used large vegetable cans in plastic baggies so as not to mar the fabric. That was better, but not great because they were hard to manage with my arthritis. My DH suggested using my hand weights and that was the perfect answer for me. The shape of them makes it easy to move them and the weight 3 to 5 pounds are not too heavy.
Another problem was that the rotary blade was not smooth even though it was brand new. I bought another and it is fine. I hope someone's suggestions will help you end this frustrating issue. :)