It is possible that you don't have even pressure on the ruler. I find that I MUST be positioned (either standing or kneeling) over the ruler and look down on it to get the best leverage. I spread my hand to get good coverage on the ruler (making sure NOT to have any fingers peaking over the edge) and I make an even, deliberate cutting pass along the ruler's edge. With a new blade and even pressure the cutter should not skip.
Good luck and be patient with yourself. (The cutter beats scissors on straight cuts any day)