One of my poults isn't growing up

[Themagicone]

Out of 10 chicks, I have 5 left. One of those 5 isn't really growing much. The other 4 are nearly double the size now. The chick eats, drinks, doesn't have a pasty butt. Their going on 4 weeks old now. Any idea?

 

[oldhenlikesdogs]

What are you feeding? Are you providing extra heat? What type of bedding?