Red raw and few feathers on bum

Hi, I am in Australia and recently bought 5 healthy, approx. 2 yr old laying hens (prob. isa browns). One of them came to me with a red, inflamed rear end and the few feathers that are there seem to be stuck together. They've all been laying fine since day one (ranging 3-5 eggs per day) but I've now noticed a couple of the other hens have the same redness showing and seem to be losing a few feathers. It could be a health issue OR my hunch is that the coop they came with is set up so they are roosting on a shelf (not a rung / bar) and each day there is a build up of manure there - if I don't clean it off each day (which I try to but ... working mum etc etc) , I imagine they are sitting on their own poo at night and this might be causing some kind of ammonia burn / irritation? I have an idea to introduce a rung perch but know they grown up in this coop and might only be able to settle on a shelf now.... Any ideas?