How old are your hens? The older the hen, the thinner the shell. Even with the addition of calcium to the diet, older hens, even those only 2 years of age, lay thinner shelled eggs. I find that reducing the carbs in old hens' feed helps strengthen the shells. I've written about that and other things I do for my old girls here (I have a hen who is 8 who laid a fine egg this year!)
http://hencam.com/henblog/2011/07/thin-shelled-eggs-old-hens-and-the-miracle-diet-cure/
http://hencam.com/henblog/2011/07/thin-shelled-eggs-old-hens-and-the-miracle-diet-cure/
