So sorry about your chick. There may have been something wrong inside, it happens. Those chicks fail to thrive for unknown reasons. It's always sad to lose the little ones. :hugs
I'm sorry no one answered before, how is your chick doing?
Was this chick hatched/incubated by you, or purchased?
Shipping stress happens a lot. Pasty butt can be from stress, being too hot or too cold. You have to be vigilant about cleaning them up until it resolves. If they get blocked...