I purchased 6 standard day old chicks from the feed store. 2 light brahams ,2 buff orpingtons. 2 blk sexlinks, and 2 E.Es. They are now around 2 and a half months old. They all have feathers, and are about the same size except for one of the orpingtons, it has not grown. It has just the same feather development as the others but the size is the same as it was went I got it. It seems to be healthy, eats and drinks, but does not grow. What is going on?