It might help if you post some photos of your girls. It may be that they have a reproductive issue going on. What breed are they? High production birds like red sex links and production reds are particularly prone to such problems once they get beyond 2 years old. Infections of the oviduct and internal laying can both cause abdominal swelling, feather loss and eventually death. Usually they will have soiled feathers below their vent, but if they preen them to clean it off, they can become bald as the poop sticks like cement and feathers come out before it comes off. The skin will often be inflamed from the poop aggravating it. Does any of that sound like a possibility? Usually they will walk with a wide stance when these problems are occurring and the abdomen becomes more swollen. Cupping your hand between their legs on a night when they are roosting and comparing one to another should make it obvious if there is something like this amiss.