This is interesting, what kind of shad are they do you know? How big is the pond? How old is it? When you say there is plenty of forage, is that by what you see when walking around? or you often catch panfish and other forage fish on rod and line?
An 18 inch bass that has to run around catching minnows is not going to gain weight. He will want (in a perfect world) to eat ~6in BG or ~9 inch shad or even ~10 inch bass. Do you feel you have that size of those fish?
Another suggestion could be to start pulling out the fish with oversize head to body ratio, and see if condition of the others improve.
Tell us a little more, and maybe we can get some help from the experts
