Since it sounds like you're nearing the end of your rope with this thing, what's to lose by completely disassembling the knotters to make sure there's no sheared pins or woodruff keys anywhere? On my old 68, I had a sheared woodruff key on the the right side knotter that would make it work in kind of a random fashion. Drove me nuts for a while.
Sounds like you've tried about everything else. You've checked your hay dog springs and all right? If the flakes aren't being held properly in the chamber and are springing back, that'll prevent proper knotting too.