The bird might have gotten into something the others have not. If the others are healthy, I think you can tell yourself you have done every thing you possibly could have done. Sometimes chickens develop incorrectly. The problems you are having now you might have going on and off for the rest of the chickens life. You need to decide if this is something you are willing to do. Do you want to take your time, energy, and care from the healthy birds to tend to the sick? Chicken keeping should not be stressful, it should be a joy. Chickens do get ill, but, weeks of tending to a sick bird is a lot to ask of yourself.