Welcome !
Your description of the problem is a bit too general with not enough detail. Lets start from the beginning. Does the twine/string load OK on both twine disks and is held in place tight? When the bale is 1/2 formed, is the twine tension the same on both sides? If you trip the knotter drive manually and roll it thru by hand, do the tucker fingers swing the twine over to park it in the twine disk? Do the billhook jaws open and close as thet twist ? And do the wiper arms slide across the billhook jaw to scrape the knot off? If any of these questions gets a "no" answer, there is a problem. A picture or two of the knotter area during all this would be handy as well as a description of the condition of the twine end (cut, sheared, stretched or just all balled up?) when the knotter drive stops.
Now this may seem very far fetched, but has anybody considered a Skype video call to look into this? I wouldn't mind giving this a shot ! The video calls are free. With a wireless connection and a good camera, I think its possible to do remote baler fixin' using a laptop and the machinery parked near the house !!!