I had a similar experience with one of my chickens. She grew feathers on her back and wings and then just stopped developing at a few weeks old. Her hatchmates stopped tolerating her and began to attack her when she came near them. We separated her and tried to house her with some bantams, but after a few more weeks the same thing happened. I think that the other chickens could tell that something was wrong with her and flock instinct took over and saw her as a threat. I kept her in a large cage by herself, but her health began to deteriorate quickly from that point and her quality of life became very poor. She was a favorite of my husband's, but none of the treatments we tried made any difference. I eventually culled her myself. I will admit that I cried like a baby when I did it, but I know that it was more merciful than just letting her suffer until she died on her own.
Quail are very social birds, but can be quick to attack anyone they see as weak. It is instinct for many animals to avoid, drive off, or kill other sick animals. It is your bird and you are responsible for its quality of life. Do whatever you think you need to do.