Do you have oyster shell out for them? I feed my flock Flock Raiser or an All Flock type of feed feed and have oyster shell on the side. If you only have a few chickens, you can give all the laying aged chickens a Calcium+D supplement once a day; it won't hurt the ones that aren't laying shell-less eggs.
It might not even be a calcium issue, but that will probably help. If you are feeding them treats, I would probably cut back and just make sure they are getting a complete feed with all the nutrients they need. I'm guessing they are newer layers that haven't worked out all the kinks, or older layers.
I hope you can figure out which one it is!
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