In addition to your recommendation, I'd check the amount of slop in the mounting of the twine fingers that push the twine into position. At least on Hesston 2/3 string balers, those twine fingers are mounted on a steel bushing that is around the mounting bolt. When the hole in the twine finger wallers out, or the bushing gets thin and dubious, the twine finger has a lot of slop in the arc it must travel to push the twine into place, resulting in unreliable knots.

Edit: one more thing: check the needles to see if the string has cut a deep, narrow notch through the iron or cast steel. When this happens, the edge of this notch can become sharp and nick the string as it is being pulled by the knotter. I would use a brazing rod and a gas torch to braze these slots cut into the needles shut every season, then grind down the excess braze material with a die grinder.