If your kitten has FKS there isn't much you can do. So many things are at work here, genetic problems due to outside breeding, whether or not the kitten got the crucial antibiodies from the mom's first flow of milk, whether or not the momcat was sick- how difficult the birthing was- all sorts of things. All you can do is the very best you can, and keep your hopes alive. I have had them do really well and then at 8 weeks, suddenly they have passed. I do not breathe a sigh of relief until they pass their 12 week birthday. Even the most skilled rescuers sometimes lose against FKS, as one woman told me long ago- "It's just nature's way of making sure only the strongest survive."