It could be the roller cutter. Sorry, but cheap ones will roll if you loosen the blade. Get a better one that the blade will not wobble. If it is a cut that I cannot use the strap cutter, I will use a 36" aluminum ruler. If careful, you cankeep a straight line bearing against the ruler and not sheer any ruler. I seem to do a very good cut if I use a #2 pencil and scribe the straight line. Patience, patience,.............