Your chicken could be sick or it might have found its way to the bottom of the pecking order. I know with my splash silkie she has always been on the bottom of the pecking order. While my porridge is the top hen. But when I had more silkies I noticed my splash scarlet was self isolating and kept as far away as she could from the others as she would get bullied such as being pecked and having feathers pulled out. Is she eating and drinking okay? Walking around being a normal chicken away from the flock not looking sick, coughing, sneezing, having a pale comb or a color that is not the norm, still pooping normally? If all those are a simple she is fine other then not wanting to be around the others might just be a pecking order thing. In which case you can A. Separate that hen for I believe it was 10 days then reintroducing and she if the pecking order is so to speak redone. Or B. Give it a little while more and see if she can be accepted again or is continued to be bullied by one or more hen. Just watch for any signs of sickness as well as chickens can try to hide it and chickens will get picked on for being sick.