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Try feeding her crushed egg shells. My hens prefer that to the oyster shell. Layers of soft eggs can have problems with their shell gland, or not be absorbing the calcium well. That can lead to eggs breaking inside, and is one cause of egg yolk peritonitis. Has she lost weight in her breast muscles or had trouble walking? This can be common in hens especially after a couple of years. Have you seen egg-like material in her poop, or yellow droppings? Here is some reading:
I also like to feed them shells mixed with yogurt or oatmeal, but if I'm not the one making the eggs the shells don't always get saved.