I was going to say check to make sure you only have one blade in the cutter too. Having two in it cuts worse than one dull blade LOL.
Make sure that your rotary cutter (blade) is held up straight. If you tilt it to the side it wants to "cut into the ruler" and makes it more difficult to cut and wears out the blade quickly. It should be up butted up against the ruler, gliding against it :D:D:D