We have just recently 'evicted' a heifer from this years show team for the same reason. She was fine when young calf, but when put on feed and broken in she began to develop a pot belly. We knew it wasn't the diet which was controlled carefully, thought maybe she'd accidently gotten in calf so she got a PG but that didn't help. Meanwhile it was still getting worse. Onne of the experienced cattle man in the area had a look at her, said it is a structural fault behind the shoulders which pushes the rumen down and creates the pot belly look. On close scrutiny this seems to be the case.
Don't know if this helps, but if you have exhausted all other possibilities it might be your problem