Query about one of my chicks

I have 3 baby chicks that are 4 weeks old. I have been handrearing them for 2 weeks. Two of them are growing well and I have taken them out of their brooder and are in a small cage. But the 3rd little one is a bit of a concern. He is only half the size of the others and doesnt have much of an appetite. Has anyone had this problem before and what do you think might be the cause?