Hmm. If you are getting a beep, that tells me you are in af-s mode with focus priority.That is not the norm for bbf setup. Some guys do use it that way, but you don't get the full technique. They have their reasons for doing it. Anyway, using af-s with bbf will lock focus once achieved, beep-locked. If you continue to hold the button, focus will not update if the subject moves. You would need to release the button and press again if the subject moved. Does this sound like something that could have happened? That is, the subject moved after the beep but you were still holding the bbf on. That is my new guess, that focus halted because of af-s and then the subject moved out of focus. If you set af-c with release priority, it will follow the action and try to keep focused as long as you hold the button down. The only pitfall is a quick first shot when the focus has not had time to settle could come of not focused.