Five week mystery chick slow development

[5LexingtonHens]

I also have a runty little chicken named Sydney. She is light brahma about 3 weeks old and is smaller than the other three pullets in the brooder (about 2/3 the size). Not tremendously smaller but enough so that it is noticeable and definitely lighter weight than the others. What is really the concern is that she does not seem to be developing many feathers. The feathers of her wing tips have come in, but that's it. No feathers any place else.

Is there anyone that can advise a specific supplement or vitamin that might help her feathers come in?

Thanks!