Can anyone tell me this is?

[tacoma2]

A few of my chicks have lost their feathers on top of their back and their tail.
I thought that my roosters were were hittin them too much or too aggressively but now my jersey rooster has got the same thing.
They lose all their feathers in one area and their skin gets very red.
I've tried dusting them with diatomaceous earth with no results.
About six out of my fifteen chicks have this condition.
The girls that previously had this are all growing their feathers back now but I'm just wondering what this is and how I can prevent this with my others in the future.

 

[Wyorp Rock]

Can you post some photos?
How old are they?
Putting your location in your profile is helpful when giving suggestions

You mention this: "The girls that previously had this are all growing their feathers back"
From this comment, could they be molting?

If you suspect external parasites (lice/mites) it is better to use a Permethrin based poultry dust or spray to treat your chickens and housing. DE is not effective, imho, against an active infestation.

 

[micstrachan]

It sounds to me like someone is plucking feathers. Can you sit with them for awhile and observe? Is there anyone ranked higher than the rooster? How much protein is in their feed? Have they been confined to a smaller area than they’re used to due to weather?