Couple of possibilities....
No reason that plastic twine won't work. That's all I use in my 14t. How is pull tension? By that I mean: if you pull on the twine from the twine disk, about how much force does it take to get it to move out of the twine box? You need fish scale to measure it (15lbs)
Do you have it threaded properly? The twine box ought to have a twine routing diagram inside the cover. If not Check the manual.
Are the needles placing the twine properly in the twine disk? And does it hold the twine when it revolves?
Is there a lot of dirt around the hay dogs (in the bottom of the bale case). These keep the last flake in position so that the twine stays tight enough to make the knot.
Are there sharp edges or nicks in the twine disk that may be cutting the twine instead of it being held onto?
Is the billhook revolving? Is the billhook tongue opening to receive the knot parts?
Does the twine get pulled out of the twine disk (not enough tension).
Is there a partial knot formed or nothing at all?
Tucker fingers are the usuall culprit. Make sure they swing together and to the same maximum angle. However, twine pulling of the billhook sounds like a twine disk problem: either its not geting the twine from the needle or its not holding onto it after the needles withdraw.
Having one working is a good place to start a comparison. As you roll it over by hand, check to see that all actions happen at about the same time. If they are good, then start looking at spring tensions: twine disk holder, billhook finger, twine tension at the twine box. Keep the conversation going until it gets working. Then tell us what what the final solution.
I presume that the sisal you are using is good square bale twine. Try reversing the twine bales and see if the problem moves to the other knotter.
Gulliver is a bit far for a road trip, but you never know.... ( I'm in Brighton).
Have you checked my knotter video on YouTube?
