It seems to be critical whether or not they got colostrum.
And I don't know what else, since once year I had three Herdie pet lambs, they all got plenty of colostrum, I raised them to a good size and they seemd fit and healthy. Two were 'loved' by their mum though she couldn't feed them, one's mum rejected her and she hung around with the other two - their mum let her.
I sold them all at about 6 months to someone who was starting out. The next thing I'd heard, a few months later, was that the 'true' orphan had died.
So maybe it's just very stressful being an orphan and it makes them susceptible to whatever comes along?