Try a left handed rotary cutter, or the orig. Olfa can be used left or right. The blade HAS to be against the ruler to cut correctly.
Important - DO NOT use the lines on the MAT to measure. Use only the rotary ruler to measure your cuts.
Your ruler may be slipping too. Add gripper dots or strips to the bottom. There are demos online or ask your LQS to help you. Don't give up ! :)