There are different ways for twine to break at the knot and several potential causes. Step 1 is to determine which twine is breaking, the one held in the twine disc or the one brought up by the needle. Check the knife arm hook for deep grooves or rough spots. Loosen the twine holder spring retaining bolt 1/6 of a turn. If you do not have am OEM operator manual for your baler, get one. The knotter troubleshooting section and systematic knotter adjustment procedures are worthwhile.