Here's a good read-through to learn more about sour crop/impacted crop and related issues:
https://www.backyardchickens.com/articles/impacted-slow-and-sour-crops-prevention-and-treatments.67194/
and another one...
I've not heard of a chick that young getting sour crop before, but if the smell is bad enough it might be, perhaps some ACV with mother would help her, and - believe it or not - miconazole, found in the ladies' needs section, for treatment of yeast infection. I fear, though, it's a sign that...
There is often one in a hatch that is the runt, that doesn't grow very fast, that sometimes seems to just weaken and die. It's not that unusual. Be that as it may, the nutridrench is a good idea, especially if the little one appears to KNOW it helps him or her. They often can notice the...