I give my chickens oyster shell supplements and most of our eggs have nice sturdy egg shells but at least one of my hens (out of 16) lays very fragile eggs. Her eggs often get broken while other hens are laying eggs. I don't think it is intentional because the shells are all there, uneaten. Half the time I am able to collect the thin shelled egg but really notice the difference when I am cracking eggs. Some take a good whack but these thin ones just fall apart. It seems this hen is not eating the oyster shell. How can I get her to take in some calcium? Any ideas?