Their diet is perfectly fine, they only way they wouldn't lay from diet is if their diet was so lacking that it was all they could do to stay alive, such as an all scratch diet.
How very wierd.
Have you checked for hidden nests or done vent checks?
What is their diet, exactly?
The 4-5 year old ladies have like hit henopause since the weather has been strange, they might not have had the hormones triggered yet or enough to continue.