My last hatch ended up being brother/ sister on accident, due to losing the first set of eggs and my broody momma needing a diff set of eggs;
One is ok one died due to what i strongly believe was a bad heart valve.
I thought at first that she had lung problems. Initially she was pooping fine but puffed, then it progressed to a lung crackle, then a popping sound. Very loud. I tried antibiotics to no avail. When i looked up the symptoms a post very similar to mine popped up and that was one of the suggestions. The other possibilities were aspiration and bacterial and fungal infections of the lungs, but again pooping normal and she was being raised in my house by a mom. And a heat lamp at the end because she wasn't making any body heat. The big red flag was her stopping her growth at 4 weeks, feathers and size both.
I haven't found my porcelains to be particularly hardy. Momma is a booted bantam porcelain, the other two are breeder stock. ????? mom had to be nursed on sugar water to keep her alive when i got her home, and the other two were very "quiet" and lethargic acting chicks next to the other ones. (EE's)