Good. Thanks for the photos. I was thinking it was on the back of the neck so this helps.
Here is some things I would suggest that you do:
-Tonight just before it comes time that they go into the chicken house (probably just around
dusk),
sit out there and observe what's going on. See if there is any feather picking going on
while they're getting settled on the roost as this is when it is most likely to occur. You don't have to do anything at this point, just observer and see what the behavior is.
Do this for the next couple of nights. This will give you a better picture of what might be happening.
-Tonight (or some night as soon as possible)
after it is dark and they are on the roost, go and d
o a thorough exam. It's easier to get them off the roost in the dark as they can't see well enough to run off

.
Hopefully you'll have a helper but you can do it on your own. If you have a helper they can hold her while you examine. (This really is easiest to do in a darkened area ;0 ) Otherwise...Grab her and stay in a dark area. Find somewhere to sit down and lay her on her side in your lap (easier said than done but just keep a good grip on her). (Use your flashlight to see.) Part the feathers right down to the skin and take a look at the condition of the skin. Look
around the vent, under the wings, and up around her head/neck/ears. Look for any signs of mites. Look for signs of lice - these will be whitish clumps around the bases of the feather shafts and very often near the vent. See what the vent looks like. Do as thorough an exam as possible and see what you see.
It is possible that she is doing this because of mites or lice.
It is possible the other chickens.
It is even possible that they could use some extra ANIMAL BASED protein. I usually use some ground meat in small pieces to feed a couple times/week.