I have yet to cut a shelf into any of my selfbows. It probably helps getting used to shooting them but I have gotten used to no cut in shelf.

Anyway. I agree the shelf cut in was my first thought about why it happened. But I agree with Arther I would wrap it. I would suggest sinew if you can. It shrinks some as it dries so it gets tighter as it tries. But other materials work too.

Good luck