Also make sure that the knife is sharp that is suppposed to cut the twine - and properly adjusted. The knife arm is maleable iron, so it can be bent to adjust the knife to the proper clearance to the knotter. Also check the clearance of the wiper arm; that is maleable iron also so it can be adjusted with a couple of crescent wrenches.
If there are any old guys around that ran a binder, they might be able to help you get the knotter adjusted right. You might even look closely at the knotter that is working right and see a difference for the other one not tying right.