I've got a NH 269 I bought used. The first year I fought tyeing problems all the time. It was either missing the knot or knicking the twine behind the knot and the twine would pop when you picked up the bale.  

I replaced the bill hooks, twine discs, and knife arms and had a good NH baler guy come out and make sure things were where they should be. Every since then I hardly have any issues with knots or breakage.

I will second Russ' observations though about natural twine. I hated the stuff that first year thick, thin, inconsistent and it was even NH twine. I switched over to plastic Orangeline 7200 with 170# knot strength when I put the new parts on . Sure it might be a bit more abrasive on wear parts than natural but I don't have the problems of twine popping anymore like I did with natural twine.

Not sure if you have a book for your machine but here are a couple pics of the pages from my 269 book that describe knotter problems.

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P9060004 (Large).JPG (49KB - 1292 downloads)

P9060005 (Large).JPG (52KB - 1110 downloads)