That is weird. Look if they are eating the oyster shell scratch. I have 24 chickens and all lay perfect eggs even though I never have given them any sort of calcium supplement. Well there is one thing and that is that i have always got 100% hatch rates from them.
Is it acting weird? I think it is sick. Try to follow it and see if it is eating. Other than that I have no other explanation. Wait and see if the shells harden in some time.