chick 3 weeks old

Got 5 Lavender Orpington chicks when they were 2 days old. One of the chicks had a vent that I think was prolapsed from the moment I got her (online, overnight shipped day of birth.) I did all of the things that the feed store, this site and other forums have recommended. (soak bootie, witch...

One of my chicks is 3 weeks olds now and barely grew at all. Sure, it gained more feathers but it's size hasn't considerably increased. What's the cause? Is it a parasite? Is it just sick with something, or is it just naturally weaker than my other chicks. It had a very irritated vent in the...