8.5 times out of 10 people will not spend the few bucks to get their chickens tested until they run into serious problems. 9.9/10 times those same chickens have a worm overload.
Have you ever wormed them?
Have you checked for mites?
You may find this article helpful:
https://www.backyardchickens.com/articles/why-are-my-hens-not-laying.64564/