I used a bell in the first two seasons. It kept falling off as she would go into thick brush. Sometimes she amazed even me, slipping thru such stuff which looked like a cantalope wouldn't fit. Anyway, I keep going to store to buy another bell for her. I finally got disgusted and gave up.
I have had the same problem so I riveted them to the collars. I put a different bell on both the collars so I could tell which dog was which when they ran through that thick stuff. No problems with the bells breaking off yet.