Hi there. This usually means a low grade parasitic worm burden- in most chickens they have to have a lot of worms before it makes them ill.
I would thoroughly worm all your chickens with a 7 day in feed course of 'Flubenvet' (fenbendazole). This will eliminate any worms they may have.
Best wishes
Andrea
Bird Veterinarian
15 years in companion animal practise, BVM&S degree Edinburgh university