You won't typically find the cause of cut strings by turning the knotter over by hand. You need to have the tension of the bale pushing back on the string to get the tension that causes the cut.

There are two places that I found cause cut strings:

1. The knot wiper develops a burr.

2. The needle gets a slot cut into it by the string passing through.

When you have a cut string, look at where the cut it, and on which side of the knot it is. Now stop the baler, turn the baler over by hand and see where the string is in the knotter while the knot is still on the billhook. Carefully observe what was touching, or is touching, the string while the knot is on the billhook. This will give you a starting place to look for what it cutting the string, because when the knot is still on the billhook, you have the maximum tension on the string on both sides of the knot and the best chance for even a slight burr to cut some or most of the string.