As you know, chickens are experts at hiding health problems as not to be eaten by predators...instinct for survival. By the time we see symptoms of whatever the problem is, it requires treatment. Otherwise the chickens health will deteriorate and/or will have to be culled to protect flockmates.
If you are reluctant to use a wormer, at least put fresh fecal samples in a ziplock bag and take it to a vet and have them checked for internal parasites.
That is a thought. Take stool samples to check for worms. Do even people who go organic deworm their chickens usually? Any thoughts? I'll call the shop too. B