Did you cull the chick? It sound like an upset belly or they are cold. Just my opinion.
I would check the heat. It’s odd you mention you’ve often had chicks with pasty butt. For me it’s only when I have had large hatches at once.
I’m so sorry for your loss.
You did everything you could, I’m sure of it. Sometimes this little beasties have neurological or internal issues we can’t see.
((Hugs))
My girls don’t roost.
Is that common for Red Sexlink chickens?
They’ve showed not interest.
And, they sleep on the floor.
not in the nesting boxes I have supplied.
Iris is a 4 lb Longhair Chihuahua.
This is her sister, Nacho ( 6 1/2 lbs.) Iris got the looks in the family but Nacho got the brawn and the brains of the two siblings.