I am only having one hen that has very thin shells, when the other hens lay around it they break it and then it's a tasty snack. Why would only one hen have the shell problem? They eat feather fixer and I haven't been giving them scratch for about a week and it isn't helping. I will get oyster shell for them soon, but how can I make sure she eats it? Do they just know they need some? I do know which hen it is for what it's worth.