Sick chick

Hi everyone i’m a first time chicken owner. All of our flock has grown nicely besides one chick. They’re all about 7 weeks now and they’re all about double her size. She has feathers but only on her wings and she’s stopped eating and drinking as much as she used to. Now she’s also been getting pasty butt it seems even though she’s 7 weeks. Does anyone have any suggestions? I want to give her a good chance! :)