What type of oats? What type of greens?
If a chick has been sickly since hatch than it's possible it has some sort of birth defect that keeps it behind the others.
Oats and greens aren't a good diet for a 6 week old chick. A good fresh starter is best. Chicks should get only the starter for at least 8 weeks to avoid problems. Feed should be out constantly, and multiple feeders should be used so all birds can eat.