I have a hen or two that lay eggs with no shells. I was sure that if we had heavy rain, then the following day I get 1 or 2. Rather weird when you try to pick them up. Suggestion on the calcium deficiency, If you feed mash then scrape a cuttle fish bone, with a knife into the mash mix. I feed pellets and just scrape it in to the feeder along with them. I use a serrated knife and its allmost like chalk dust. I have plenty of it as my Canaries and Australian finches have it available 24/7. You can get it from any pet shop and its not expensive at all, I'm no expert, but it is pure calcium, got to be worth a try. I know that in its whole state nailed in the feeding area, they don't look at it.
Craig