Sorry, I didn't mean to suggest in any way at all that you didn't feed your hens well. It's just that when one starts feeling ill, they sometimes don't feel confident enough to muscle in with the others to get enough, or they appear to be eating but are not actually swallowing anything.... perhaps just picking up food and dropping it again.... but gradually they get weaker and can't pretend to be normal anymore and just huddle somewhere. By then, they have already eaten into their body reserves and lost condition and have nothing left to fight the illness..
Anyway, it's good to hear that she has perked up a bit after her bath and great that you are already familiar with tube feeding.
I have Marek's in my flock so I know how difficult it is to have a sick bird and to hope for the best but know that the worst is more likely to happen. All you can do is do your best for her and make her as comfortable as possible.