I have a similar issue with one of my girls and posted about it here a few weeks ago. She started in late June and I thought it was the molt before laying but she never grew them back and has not started laying yet even though 5 of the 7 in that flock are and the 6th is now squatting so she'll probably be any day now. I never really saw her being picked on until I ruled out any kind of bugs and really started watching. She is definitely low hen. Two of the others actively chase her away from treats and yesterday I saw one actually pick a feather right off her saddle. Such mean girls chickens can be!! I have been taking extra time to be sure she's getting her fair share of the high protein snacks like meal worms and cheese (instead of just hand feeding or scattering them in one area, I do multiple spots so if the mean girls chase her away she can find another spot) and I watch her to make sure she's getting some, call to her to show her where I have some for her, etc. It seems to be helping because I can see the feathers trying to grow in but she's still an awful lot more bare than I'd like her to be for as cold as it's starting to get here. If you've ruled out bugs, maybe just watch them for a while and see if she isn't being picked on in more subtle ways you might not see if you aren't looking for it.