Yes, I thought about a similar scenario. After an hour or two of waiting, he's daydreaming, eating a sandwich, etc.... Suddenly, police motorcycles appear and then there's JFK's car now coming towards his position. So he has to put down his sandwich, maybe quickly wipe his hands on his shirt, adjust his body, pick up his rifle, steady himself, acquire the target, etc... It still seems like enough time for 1-2 shots, but perhaps by then, Kennedy's car is already starting to turn (??).
I dunno, and I'm not convinced its a conspiracy either. Just seems to me he passed up what was plenty of time for what were clearly the best shots, only to wait and take the much more difficult ones.