While doing a bit of research on "Fading Kitten Syndrome" or what some of us call "failure to thrive" in kittens, I came upon an interesting article posted on the Winn Feline Foundation's web site. The report is a little stale, but it may explain why some kittens just don't make it and we can find no obvious reason for this ... the article is a little technical.
http://www.winnfelinehealth.org/reports/blood-type.html
http://www.winnfelinehealth.org/reports/blood-type.html