Yep check the blade for, chips, could have been dropped or running over a pin, (how would I know that) also grooves in the cutting board. Practice made a big difference tho.
One more thing, when I learned, left the part I was cutting off exposed, so could softly scoot it over to see if all was cut, without letting up on the guide, but the grid was straight, can cut both ways now with guide over the strip I'm keeping.
Don't think I've ever cut me tho ;) did cut the phone cord in half the other day :(..... Woops :)