Blue Green Chromis acting strange or sick?

So I've had five blue green chromis in my 75 gallon tank for about 6 months. I've heard of a sort of pecking order that they display in groups but have never noticed very much aggressive behavior between them. Recently one of the chromis dissapeared (one of the larger ones). Now one of the remaining four only stays on the bottom of the tank away from all the others. The other chromis don't go near him or show aggression towards him and he is eating if food ends up at the bottom. I thought that it was usually the smallest chromis of the group that would get bullied but this one is average size and there is one other chromis that is much smaller than him. The chromis is still eating but I don't know if this is sickly behavior or the other chromis trying to starve him out. With one of the chromis already disappearing mysteriously I'm wondering if the two events are connected. :confused: