I doubt that it's a problem with calcium intake, especially if you have other birds that are doing fine on the same ration. Shell-less and very thin shelled eggs are usually caused by the hen laying the egg too early, before the process is complete (or possibly a bad shell gland). For what reasons they do this, I really don't know, but it's not due to a lack of calcium. Feeding them extra calcium isn't going to make them add a shell. We have 2500 hens and I typically find those kind of eggs laid at night, off of their normal laying cycle. 80% of our eggs are laid in the first four hours of the day and the other 20% within a few hours after that. Very, very few of our eggs are laid in the afternoon or the evening, yet I will walk through the barn late at night after they have gone to roost and will find a few of these on the floor, laid in the first few hours after the lights go out. Sometimes I believe it is from me disturbing them at night. I will walk through the barn with a flashlight, pass a group a hens, continue around to the other side of the barn, and on my way back will find that a bird has just laid a shell-less egg, almost as if I had scared it out of her. Whether that is possible or not, I don't know...