Teeth might also be a good area to look at in terms of figuring out how it works. Since most people already grow two full sets of teeth, and I believe there are numerous cases on record of people who have grown more than that, perhaps the regeneration genes are already partially activated in that region of the body.

Regenerating body parts doesn't happen because it is hugely expensive in terms of biochemical resources. Teeth are the most materially expensive parts of the human body. That is why teeth are often the most long lasting and best preserved human remains.

I believe the hairs within the ear also don't regenerate (leading to deafness when damaged through loud noises).

that was my first thought as well ( I just had a bad dentist experience... )