Are you putting the electrolytes/vitamins in the water? When I ordered mine and they arrived, w/in the 1st 3 days we lost 5 from the stress of the trip. I started adding the electrolytes and vitamins in the water and didn't lose anymore.
I had almost the same exact thing occur.. my drake was pulling out the feathers of my hens and injured the ones vent pretty severely. I had to separate the chickens from the ducks.
I did some research on that and inner laying, since I never got a single egg from her. And all the others from her same batch are laying every day. She was 9 months old. And was always the smallest. About 2 weeks ago, 1 of my drakes roughed her and one of my other hens up. Pulled out some tail...
Ok... looking for some ideas as to what may have happened to my hen. All the others are fine & laying everyday. This one never laid a single egg. She started losing weight & the others were picking on her. So i cagesd her up, started giving her some electrolytes & she seemed to be doing better...