My thirty years experience is confined to mainly New Holland balers but the principles are very similar.
First of all you will need to check the twine tensioner. This is normally found at the end of the cord box. Ensure that it is intact and that both cords run between the plates and require a similar amount of effort to pull the cords through. I have a feeling that this may be where your problem is. Follow the cords down to the china eyes and up to the needle eyes and then to the knotters.
We need to identify the cords now. The end that is held in the knotter and runs along the top of the bale is referred to as the number 1 cord. The cord that the needle places in the knotter is referred to as the number 2 cord. The number 1 cord is held in the twine disc/s. Compare the faulty side to the good one. The knotch should be in the correct position to accept the No 2 cord when it is brought up. If necessary empty the chamber and have an assistant turn the baler over by hand at the flywheel. Carefully holding the cords inside the chamber to simulate a bale watch the operation. The needles will come up and lay the No 2 cord in the twine disc. The twine disc will turn to grip the cord. The twine finger on the case will move to ensure that the cords remain in contact with the billhook. The billhook will turn and open and close on the cords forming the start of the knot. The knife arm will cut the cords as the knot is being swept off the billhook and the knot is formed.
From your description the number 2 cord has no tension;
Check the twine tensioner.
Check the threading of the cords.
Check that No 2 cord is being laid in the twine disc and that it is being held there.
Let us know how you get on.