you can put different colors of food coloring on their vents / but don’t use red . if you have a lot of chickens, ( and not enough colors) you can do a few each night .
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!
Thanks for joining and welcome to BackYard Chickens!
Welcome to BYC.
How old are your hens? Do you have any new layers or elderly layers?
What type feed do you use?
Do you have additional oyster shell/calcium available for them at all times?
If you can get one of those lipstick tester kits with the tiny lipsticks in different colors, you can use those to mark the hens' vents at night, then match the egg to the vent to see who laid what egg. NOTE: do NOT put those lipsticks back in the house for future human use, lol!!!