There are times some of my birds have had mini molts and lost only neck feathers. Otherwise it could be a rooster like Alaskan mentioned. Lice could be an irritating problem and your hen is scratching out the neck feathers, inspect her for lice/mites. You could have a feather picker. Have you seen feathers on the ground? Keep an open eye for the feather picker. If this is the case, increase protein in their diet. I recommend gamebird feed for about a month, then wean them back to layer feed. If there's still feather picking going on, cull the feather picker. If you dont, the rest of them will become feather pickers.