What are you feeding them?
I used to do "age feeding, due to my limit to 4 chickens I had "left over" ... Although I was told I could finish off the Chick Starter before starting Layer. After reading several post about upping the protein & egg laying, I changed to Flock Raiser (20%).
I finish a 5 - 10# bag of Chick Starter then switch to FR add a separate dish with Oyster Shells when they get ready to lay ... Strong shells & yummy eggs. I also ferment their feed for nutritional & hygiene (less poop, not stinky except the cecal, firm) benefits.
Any 'supplement' that you add to an inadequate base diet will cost more, and do less, than feeding the good stuff in the first place. You might have a bird laying thin shelled, easily broken eggs, and everyone will dive in to eat an egg that's already cracked.
If you have one bird who's breaking eggs on purpose, she may need to leave.
Bad habits can be hard to break, but it's worth trying.
Ceramic eggs, mustard filled egg shells, darker nest boxes, or roll away nest boxes, are all methods to fix this problem too.