My dad has had that happened to him a few times thru the years but only when he was drag racing. He always just swapped out the rocker for another one. Be sure to check the pushrod as sometimes it gets bent in the process. If they are originally rockers you have to remember they are old and some will just fail randomly. Cheap fit would be to put another one on and keep on trucking. It is werid that if failed at low rpms, but stuff happens.
