How long since you sharpened the twine knives? That's the first thing I'd do and if it's still not wiping the knots off the billhook, then look at the clearance of the wiper to the hook..., check the twine tension and check the billhook finger tension.
If it worked fine before and this is a new problem and you're using the same twine... I'd assume the twine knives are dull. Remove and file with a flat file.