I think the biggest problem Lesnar has is that he isn't pushed in training because of his setup, he is the boss at his gym and he makes them train how he wants to train. Which, according to various sources, includes them not being allowed to hit him. There are several reports of him whining in earlier years about getting hit in training, apparently he fired Brett Rogers as a sparring partner because Rogers KHTFO and "hit him too hard", and there was another thing with Tim Sylvia and someone else who I forget who he trained with and moaned about getting hit in standup so he quit. It's highly likely that whenever he does train standup at his gym, he is the one punching and he doesn't spar to get beat up. So he takes getting hit like most untrained people do, by running away and turtling up. Until he mans up and learns how to take a punch (not like he has a bad chin or anything otherwise Carwin would have killed him, he just has never trained to not react badly to getting hit) he will always fail against people who he can't take down at will and gets punched in the face by.