Some people put a small amount of apple cider vinegar, in the water - which is supposed to treat pasty butt. If she's pooing, she must be eating something. YOu can put some vaseline around her vent, so the poo will come off easier.
Have you gently dipped her beak in the water, so she knows where it is? Maybe the chick feed is too coarse for her and needs to be ground finer. She may also like it if it is a bit damp.
Have you tried coaxing her with finely mashed hard boiled egg yolk? Hope she starts improving soon.
Do the other chicks bully her away from water & feed, they tend to do that with weaker chicks. So an extra water and feeder away from the originals, gives her a chance to get some.