I'm new on here and need some guidance!

Hello all, i have 4 chickens, a silky a columbian blacktail, a Ranger and a big blue hybrid for nearly a year now. They are so funny to watch! But my Ranger over the past two months I have noticed is missing a lot of feathers round the back and some days can be very sore looking. They have fresh straw, frequently cleaned out and I use dust mite powder and proper chicken coop cleaning products they all eat the same thing so was wondering if she has an alergy to something? If anyone has had the same thing and could shed light on what is wrong with her. She is the most alert out of all of them the most domestic and friendly I really hope it's nothing deadly!