Oh no, so sorry she passed. :hugs
Her droppings probably meant she wasn't getting enough to eat.
I do think she was a failure to thrive, and there's nothing you can do at all to fix it. You wouldn't have caused it, its just something chicks are born with and sadly they just don't make it a lot...
Its likely failure to thrive since she's been like this her whole life, there's not really anything you can do about it.
What's her diet? What do her droppings look like?