bald hen

can anyone help me with my problem.  I noticed today that one of my 4 hens is bald on the back of her neck.  I found the feathers under a bush in the garden where she laid an egg a few days ago.  She is top of the pecking order and there is no evidence of bullying

I got the hens last august and they were POL then. They are all laying perfectly well and there are no signs of mites or lice. Do they lose all feathers?  Its quite scarey to see bald skin