How big is the coop, the little building where they sleep? Measure it from side to side and give us the dimensions in feet, L x W. Chickens typically do best in a coop that allows each bird to have 4 sq ft of floor space, so for 9 birds you need a coop at least 36 sq ft. Or 6 x 6 for example. If they have a significantly smaller space and feel crowded, that might cause them to pluck each other's feathers.
Another possible cause might be the feed. If they are not getting enough protein they might do feather plucking, as feathers are high in protein. Do they have commercial feed available all day? What is the protein content?
If possible put some branches, something in the enclosure for chicken to jump up to perch or something to hide from from the trouble maker. Chickens need places to hide if they need to.
When I divide 100" by 12 (length) and 120" by 12 (width) I get an 8x10 building. But there is a range of dimensions in that chart. So I still don't know how big your coop is. Is the coop 100 inches x 120? Or 170 x 122? Or 170x 60? It's very ambiguous based on that chart. Can you just measure your unit and tell us what you have?