Describe the condition of the missed knot: no knot, twine broken near a formed knot, knot in one side of the twine, etc.
It could be many things. Check tension coming out of the twine box. Ought to be about 10 lbs per. Check tension in twine holder: same retention force. check for play in the tucker fingers: do they clear the needles by just a small space?. Check wiper arm knot scraping force. It also could be worn balls on the end of the wiper arm. That's why it misses every so often: the ball rotates a bit after each knot. (Make sure the balls can be rotated. Otherwise they will be flatsplotted).