Maybe I missed how old your older layers are...but if they are near 18 months old they may be slowing down to molt.
So you have 2 separate flocks, one younger than the other...what are the ages of each flock?
Sometimes layer feed is not enough, it's minimum protein, especially if they are...
We've actually lost about 3 hours of daylight since the summer solstice....but yeah about 20 minutes in the last week.
First year layers usually continue to lay thru their first winter without supplemental lighting...but they may slow down.
You could still have an egg eater.
Do they free...