It sounds like you have a couple of extreme options (or something somewhere in between - that's my disclaimer)
Kill the pond and start from scratch.
OR
Start seining and removing the bullheads and any larger bass (anything over 14") Larger bass eat larger BG and keep the BG from getting big. If you have no bass, stock LMB that are big enough to not be eaten by the fish you do have. Too many BG breed way too many more bluegill and fill the pond with biomass cutting into the available food sources, stunting all the fish. So, you may need to remove a lot of smaller BG too. Removing the bullheads will also help reduce the biomass leaving more food for the more desired BG.