There was a discussion on a board I’m on concerning hens whose butts are bare. While my post on molting or pecking problems may address lack of feathers in part, bare butts may also be due to laying activity, especially in birds that lay well. This article has some good information and pictures: http://www.hencam.com/henblog/2010/10/bare-butts/
Good layers can end up with feather loss due to putting protein into the eggs rather than into keratin-rich feathers. If protein deficiency is severe enough, they may eat feathers. Always feed a balanced layer ration (16-18% protein). You can supplement protein with dry cat chow (a handful per day) or with high-protein chick starter (NOT a good idea for pullets, as this can cause them to go into lay too soon).
Another possibility is external parasites, such as northern fowl mites or poultry lice. You can manage these issues with diatomaceous earth or by spraying the birds and their coop with external parasite pesticides. There may be a withdrawal period before you can harvest and eat the eggs.
Concerning feather loss, it is important to ask, what is the underlying cause? Feather loss pattern can help you determine what is going on: aggression, parasite, nutritional deficiency, reproductive behavior, something else altogether!