Chicken bald back

[EllieFRG]

2 of our chickens have in the last month lost the feathers on their back near the tail (definitely not moulting!) We assumed they were being pecked by the third chicken as there had been a change to their flock recently.  However a few weeks after going bald 1 of the girls started limping; she is an old girl so we wrote it off as that. She then started bleeding in the same area, got scaly skin and sadly went quickly down hill and died. I am now worried it’s something contagious that could kill the others. I’ve looked for mites and couldn’t see anything, but understand they can be hard to spot and with so many different types not sure what to look for. 

Does anyone have experience of similar symptoms or ideas for treating it? They are free range in our garden, plenty of space and light, clean bedding, layer pellet diet - all the basics should be right.

thanks for your help, from novice chicken keeper