I've had soft/thin shell problems too sometimes and eggs broken in the nests. I feed "premium" quality layer food and they always have oyster shells available but it seems like they don't eat it much. I also give them yogurt and cottage cheese and still problems. I finally found something that works and all the chicken's eggs are really good now. Found it at Meyer Hatchery....BVS Vitamin D3. It is a highly concentrated liquid and I just put literally just a few eye dropper drops in a gallon of water. They drink their water so they're finally getting the calcium they need.