Happens - ive lost 3 this year - 1 to badger, 1 to unknown (keeled over about 4 hours old) and one just went down hill even on the bottle after the mother was lost (turned out to be struck) - next year you may have 50 without a loss, the year after 20 with 5 - sadly thats the way it can be!
If theirs no reason visible, it could just be she had some problem or issue internally (they can be born with missing organs, defects, weak hearts etc) rather than anything you could of prevented.
Focus your efforts on making sure your surviving stock are all healthy, well fed and happy.