? Unwell Girl

One of my girls is suffering from a fair bit of feather loss, i`m totally unsure as to why.
She has a bald patch on her vent and on her chest/brest. Although a couple of the others are in moult this poor old girl seems to be having an extreme one. There are no signs of any other symptoms, i.e. the bald areas are not red and she is eating and drinking ok
Thanks in advance
Pete Hornsby