Unfortunately, it is not always possible to get a nock low, either because of your form or because of the tiller of the bow. If you want to see which it is, you can temporarily mount an arrow rest 1/2 - 1" above the shelf, and shoot your bare shaft from the rest. If it is a tiller problem, you may be able to get a nock low shooting from the arrow rest, when you couldn't shooting from the shelf.