Chick with stunted growth. What's the cause?

[Harun]

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 past, but it's healed quite a bit now. My chick isn't lethargic, just hasn't grown much. Whatever the cause is, I hope you can help inform me about it so I can get it treatment as soon as possible.

 

[RoostersAreAwesome]

Do you have a picture? Is the behavior normal otherwise?

 

[Harun]

Do you have a picture? Is the behavior normal otherwise?

It behaves normally. When I take it outside it bathes in the dirt and has a lot of fun. In the brooder it perches quite a bit and comes down to eat, as well as sip the water from time to time. Behavior is normal. I'll send a picture comparing it to one of my other chicks