Now that you mention filling the AF sensor with the subject, the shots that I missed would have been the ones where the canoes were a long way off and they only sit inches high in the water any way so the subject would not have been filling the AF sensor
Arthur, I very rarely use live view for anything, maybe some macro stuff, to confirm framing etc, the screen on the 40D isn't exactly the best. Don't even know how to get AF to work with live view