Was it like that from day 1 or did it develop recently..... I've seen fish "bob" around for many days, (normally in all broods, it's the runts) & suddenly come right overnight, but then again there are those that just never improve. I've no other advice than to just favour it at feeding time & watch it does not get bullied away from the foods..... If it's been for longer than 2 weeks, I don't think its gonna get better.