I just went through the same thing. Failure to thrive (FTT). It's not fun watching and trying and watching and trying, but apparently there is little that can be done to rectify the little ones behavior. I tried live feeder guppies and that didn't jump start the feeding process either.
As others have said, sometimes FTT is natures way of dealing with an organism that is perhaps unhealthy in other, unknown ways. At any rate, it seems to be not that uncommon. It's sad to be sure, and I felt responsible too... but the reality is sometimes there is nothing that can be done.
You guys gotta understand that some just aren't meant to make it. Still, I suspect that some that flat refuse to eat are simply waiting for spring. Many babies are born at a time of year when appropriate food isn't available, or they are born late in the season. Rather than keeping them warm and holding out for them to eat, sometimes they can be saved by chilling them down cool enough to slow their metabolism so they don't starve. After a month or so, warm them up again and they'll often snap out of it and start eating.
Sam....I just caught up with this thread and I am so sorry to hear about your little one. I know it is hard to not blame ourselves...to wonder if there is something that we could have done. But this isn't your fault. Some little babies just never come around. Again....I am so sorry.