I've been through this with one of mine (or maybe two). I thought it might be a calcium deficiency, or re-feathering (they're all ex-battery) or something that was making them a bit run down. I gave them some poultry spice in a porridgy mash made with their pellets mixed with hot water, cooked mashed potatoes and they loved it! I also did a de-worming and a de-louse on all of them even though there were no signs of lice, red-mite-powdered the coop at cleaning time, and put some Life Gueard tonic in their water - all to make sure nothing could be stressing them. Also put down plenty of grit. Hopefully it was just a temporary phase, though I do still occasionally get a big one with a very fragile shell. The eggs are now getting a bit smaller, but more of them. I've also cut down drastically on their "treats", I did spoil them rotten and it wasn't doing them any good, so the treats are now a cabbage every 2 or 3 days (hung up so they make it last longer and get more exercise
) and a handfull of mixed seeds and corn chucked over the soil about an hour before bedtime
Good luck - I think it might be a case of "Patience is a virtue"
