So sorry to hear this, Cliodhna, after all your care. I'm not very surprised, though - once they get to such a seriously ill stage, recovery is usually doubtful. They hide the early signs of illness so effectively that often, by the time even the most conscientious keeper notices there's something wrong, they've gone downhill badly. And chickens are subject to various internal illnesses for which there is no cure.
She was a very lucky girl, though, to have had such good care with you right from the time she was hatched. It's always seems to be the most loved ones who fall ill and die.