Are they with a rooster? He could be ripping up her feathers when he breeds her.
If not, she may have ripped feathers out getting caught on something - did this happen all of a sudden, more gradually, or is it something you just noticed? Is it just a strip on her back that's missing feathers, or does her skin look cut or grazed? Is she eating, drinking, acting normal?
It's the wrong time of year for moulting, so it's probably not that, unless you live in the Southern Hemisphere? You can put your location in your profile - it's helpful to know what part of the world you live in - you'll get better answers to your questions depending on the type of climate you live in.