Multiple feeders/ports, and OS holders, are always a good thing.
3 ports(or more) for 13 birds would be good....adjust the height, or use bricks for steps, for smaller birds.
I made two of these for OS and egg shells:
https://www.backyardchickens.com/gallery/oyster-plastic-jar.7897731/
Which of those two feeds get eaten up faster?
I just feed a 20% all flock with OS in a separate feeder.
No treats except for daily minimal scratch while cleaning coop.
I do all exams off the roost at night after dark, wearing a headlight.
They are much easier to 'catch' then.
So Opal is older than the listed?
What all and how exactly are you feeding?
How old are they all in weeks or months?
It's really hard to say who laid the funkies.
You might need to stalk the nests to catch them in the act before panicking.
Some exams may be in order.
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/