Slow Feathering CornishX

I noticed one out of 150 CornishX this morning with almost no feathers. The birds are over 4 weeks old, have been in a Salatin-style tractor for over two weeks. All the others look normal. This one chick has maybe 1-inch long wing feathers, but is essentially bald elsewhere. The feathering looks like what you might see in a week-old chick. He seems healthy, about the same size as the others. Any ideas?

 

Maybe so. I've raised over 700 since last Fall and have never seen one like it.

 

They were on 28% Game Bird starter/grower for almost 4 weeks, have just switched them to 20% starter/grower. The strange thing is the rest seemed to feather faster than normal, even with 90+ degree temps.