One of my hens (a sex link cross Baltimore/Road Island Red) has been laying shell less eggs for the last 2 weeks. She is only about 8 months old. She did this some when she first started laying and I added oyster shell to their diet and it was back to shells but the last 2 weeks all of them are shell less. The yolk and white all come out in a pile all at once like she would normally- just no shell. I'm currently feeding Purina crumble for egg layers. My other one is fine but she is a Buff Orpington. Shells are thick. Could it be her breed or is there something else I can try?