Help needed feather loss

[Shaz1522]

I've 7 chickens and 4 have bear bums and chest.
2 of them even on there sides?
I've checked for mites and lice but can't see anything
There coop snd run is big and I open it up every morning so they have bottom of garden to
One chicken now has purple comb not sure if that's linked or she just has heart problem
Any ideas
I've not seen a pecker.i use to have one 6 months ago but rehomed her

 

[nuthatched]

What's their diet? If they have plenty of room and enrichment, and verified no mite issue, it could be dietary