I just found this old thread - I am having the same problem and I have just four hens. Everything was fine until a couple of weeks ago. Since then I have been trying to be diligent about letting them out in the morning.
They have two roosts - one is often alone on the one roost and three are on another to sleep. It isn't always the "bottom" hen either. Sometimes the top hen sleeps by herself on the other roost.
They have a lot of run space but I can't let them out in our yard unattended anymore because some dogs have seen the chickens and come into our yard. Plus, last summer they all started flying over the fence to our neighbour's yummier garden. Their run space is 7x13 plus 4x8.
I admit it hasn't been as fun out there until recently because the snow kept them more often in their covered 4x8 run. I did shovel their other run out nearly daily.
They can now scratch and dust bathe etc. since the weather is warming up nicely.
The lowest hen has almost no feathers on the back of her head but she is doesn't have any bleeding so I hesitate putting anything on there. Seems like most of the pecking is related to feeding.
I am wondering if the following might help:
A second feeder (the three often hog the one feeder which does have four holes, but of course they don't share nicely)
More "activities" for them to do (like pecking activities - cabbage etc.)
Switching from pellets to crumbles or something?
Thanks for any advice!