Usually missing back feathers is due to an overly amorous rooster. I just had to get rid of mine for this problem. 21 hens wasn't enough for him, he was really tearing 4 of them up.... That being said sometimes it's a numbers game, the rooster just doesn't have enough hens. If you don't have a rooster then I have no real idea what could be going on here.
You could try some Diatomaceous Earth for the bugs. It's a powdery substance you get from the feed store that you can spread in their bedding on directly on affected birds. It basically cuts the bugs up on a microscopic level but shouldn't hurt the chickens any. I've been using it with great success for a while now. Good luck!
They are all different ages. Will take pics when I get home tonight. I have one rooster and 27 hens, but he's only servicing 5-6 of them.the others were added later and have always kept to themselves. They are all free range( so I'm loosing eggs somewhere on my property). Maybe lice or mites? Was mostly on their backs and I figured it was the rooster but when It started on their necks I knew something was up
You can examine their skin around the vent area at night, using a bright light, and be able to see if they have mites and lice. There will also be eggs at the base of the feathers if so...I can dig up some pics from another site of these eggs if you don't know what they look like.