I'm a great believer in food. I don't know much about what your hens get to eat, but even here free ranging some of the hens, particularly the bigger ones seem to do a lot better with some added vitamins when laying.
Some people use supplements which is fine. I prefer whenever possible to use proper food and keep the supplements for when a hen has lost interest in eating normally.
Fish and meat have done wonders for perking up hens that while no clinically sick, are not quite themselves.
Some of the healthiest hens here hardly eat any commercial feed. Donk for example forages most of what she eats and tops up with supper time treats.
I should point out that supper time treats are in general fish, meat, walnuts, sunflower seeds, cooked rice with coconut, tomatoes, grapes, occasionally cheese, yoghurt, whole meal bread, wholemeal pasta but mixed together in various combinations, never just one on it's own.
I could live on that while I couldn't survive on the commercial feed for very long.
For most creatures, diet and exercise are the building blocks of good health. This doesn't mean one doesn't get sick, but it can mean the chances of recovery are better.