My Oliver 520 had a similiar problem, twine would break every few bales. Same style of knotter as your Deere. Upon investigation I found that the tails of the knot where getting hung up in the billhook jaw, and the twine would break as the next bale was formed. It would either pull apart the knot or break right at it.
I did all that everyone above suggested, wiper arm adjustment, twine tension, checked twine disc timing, and bill hook tension, etc.
The problem ended up being TO MUCH tension on the bill hook. I backed the nut all the way off and turned it in a turn at a time until I was forming a good knot again.
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