Just to rule it out, why don't you search up a YouTube video on chickens preening, just so you can see what it looks like. Healthy chickens will do it daily, and will likely do it more if they're feeling a little more uncomfortable from molting.
I don't know how long you've had chickens, so I don't mean this question to be insulting, but do you know what it looks like when chickens preen themselves? It could be that they're not actually itchy but just trying to groom.