Yes, try a bigger ruler. Check that the mat is flat and level on the table. Pay attention to technique. Are you sliding the ruler onto the fabric? "Lay" the ruler onto the fabric. Also check your hand position on the ruler. Spread out fingers and position on the ruler. Great demo link below for you.
https://www.youtube.com/watch?v=XfvKE0Y6npk
Be sure the ruler line is "over and on" the fabric. If the line is only to the fabric, the cut will be short by a thread or two of fabric.
https://www.youtube.com/watch?v=9bkJKQwq4FI