When you are first learning how to rotary cut, it is easy to rock your hand slightly forward or backward. Any variation in the pressure could account for a couple threads not getting cut. I would find an old shirt or rag and practice applying even pressure, making a smooth stroke. Also, if you start your cut an inch or so in from the edge, cut back towards you and then away from you you can get a smoother cut. It takes a little time to get the feel of it. Once you do, you will be off and running!