I think people don't understand what stimulates the chickens. I've learned alot, just having chickens.
First of all, its their response to the length of the day.
The longest day of the year is June 21, then they start getting shorter. That's a chicken's stimulus to slow down laying. The shortest day of the year is December 21, then they start getting longer. It is at that moment that the chickens' bodies realize "aha! The days are getting longer once again, so its safe to start laying eggs and raising babies."
Younger hens seem to really respond to the days getting longer immediately, so they start putting out eggs soon after December 21.
As my hens got older, they would really slow down egg production in August/September and were really slow to pick back up in Jan-Feb. But younger hens definitely respond to the increasing daylight. Your chickens are absolutely normal!