If you are not seeing evidence of mites or worms what makes you think that is it?
could it be cloudy where you are and not filling their sunlight needs? Or are they staying in a closed up dim coop?
is something stressing them at night? mice rats or other critters?
Have you made any feed changes?
How old are they?
I am not trying to be an expert but sometimes we can over look things we already know thinking it cant be that easy.
I admit sun has been lacking lately, but it is so every spring. They have plenty of space in a coop with multiple windows, and now that the snow is gone (oh wait, its coming back) plenty of room in a run, plus they get out and free range when I am home (also now that the snow is gone except for the 10-14 inches coming down now).
I did have some problems weeks ago at night when they could come off the roost at night and pile/huddle in a far corner. I never found evidence of any critter- no tracks in the snow, no droppings, and this behavior stopped.
Am always messing with their feed. I have never been a believer of conventional wisdom that for chickens, dogs, cats, they must be fed a repetitive diet and any changes dreaded and introduced slowly. Have never had a chicken, cat, or dog, become sick, upset, or disheartened by the introduction of a new feed.
Mostly they get an all flock mash supplemented with greens, leftover, etc. I have been getting rid of a 50 pound bag of organic feed, which contains an abnormal amount (to me) of flax seed that they have refused to eat. I add a scoop to the feed each day, which is moistened with water,milk, yogurt, cooking water , etc.
The youngest are less than a year, the oldest is 5.