I think he lacked the physical tools to adapt to certain styles, he wasn't remotely "weak" or anything like that. He was a great accumulative puncher when he got into a rhythm, but since he wasn't nearly as fast or slick as some he sort of died by the same sword once he was getting beat to the punch. Perhaps he wasn't always in top condition, and he didn't have a rock solid chin either, but there was never a point in a fight where he stopped trying to win imo.