You may have nicked the blade on the ruler. I've already put in a new blade and accidentally ran the blade over the edge of the ruler and nicked it. That happens if you have a pin in the fabric somewhere and run over it. Guess who has done that before?? That would be me.
I use the Martelli cutter and since switching to that have no trouble getting off, when I cut. Also you might try getting one of those handles that clamp down on the ruler, mine is actually a suction type one that I got at the hardware store, they are supposed to be put on a shower wall to give a hand hold there. It helps to keep the pressure consistent on the ruler. Hope that helps.