Sometimes adult fish don't do well when transplanted to a new BOW. They aren't used to the pond, and have a hard time finding food in the new environment. Also, if the "new" (to them) pond is new, then there won't be the forage base that is built up in the old pond.
Remember, different sized fish (even of the same species) do better with different sized food. It's hard to get fat when you are chasing tiny morsels all over the place vs. sitting down to a big meal.
