Sugar. The answer is sugar. Dogtooth's "Clairvoyant" Observation Haki works because he's pushing his brain to it's utmost limit. This is not only tiresome, but in order to keep using this power, he needs to refuel on sugar. L Lawliet from Death Note consumed insane amounts of sugar in order to keep his brain functionality optimal. Sugar is good for the brain (not TOO much). Same thing applies here with Dogtooth.
The reason Luffy go his hit in is because Dogtooth didn't get to eat enough sweets to get his That's So Raven powers up and running. He also stated he was stressed and irritated. Why? He pushing himself too hard and desperately needs to finish off Luffy so he can eat. What do you guys think of my little theory here? Yes or no? And why.