I have gotten some of those paper shelled eggs from my Comets. They are always out of the nesting box, so I assume it feels more like a poo to the chicken(?) Anyway, the first time it happened, the vet told me to put out oyster shell. Something my original chicken information source didn't tell me about (feed store where I got the pullets). So we did, and the chickens do go for it pretty good. But we still, very occasionally, get one of these eggs. I woulds love to hear an explanation of what and why.
Oh, and as for your oyster shell size.... have you dug to the bottom of the bag? We get the big pieces on the top and the finer pieces drift to the bottom. But my chickens like the bigger chunks. They quit eating it when they get to the bottom stuff and I have to get them more. We keep a heavy dog bowl full of it.