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If they have regular calcium (oyster shells), other things to check are
1) are they on a layer diet? (Is your food for laying chickens)
2) has there been any unusual stresses lately?
3) if she is younger or older, the eggs might be deformed
If none of this applies, start looking for other things that could have disturbed her. Is everything in your coop relatively clean? Does she have symptoms of disease (Discharge from eyes, losing feathers, ect.)? Is she getting bullied by other hens/roos? Look for things like that. If there is absolutely nothing wrong, it might be just a cold, or she might have worms or another disease. Check her feces for worms. (Also, this is just what I have read, because I just got my chicks 1 week ago, but this is accurate information from a lot of sources) Hope this helps!
Edited by peopleRanimals2 - 4/17/16 at 11:30am
I was about to ask the same thing. Sometimes older hen's shells can end up thinner and easier to break. I don't know if the fact that she is small and lays big eggs has anything to do with it, unless they are unusually big and she usually lays small eggs so that if she laid an extra large egg or double yolker they could end up having thin shells because the material it usually takes to cover her eggs has to be spread over a larger surface so it ends up thinner. If that makes any sense. Otherwise, does she eat the oyster shell and eggshells? Sometimes one of our hens are stubborn and won't eat it, then you can try mixing a little calcium powder or really finely crush oyster shell and put it on a special treat or something she likes to eat if you feel she isn't getting enough. Hope at least some of this will help you. Good luck.