Perhaps I'm reading too much into it, but I really think his career from the time he was made Captain was never really at the same level again.
I'm not sure if that was coincidental, or just that his career slowed down as he got older and slower (he was never really in amazing shape) but I felt things started to go poorly when he was made Captain, then only continued to dip in form and consistency overall once that was removed from him again. It was almost as though the weight of leadership when he was in the job, then the loss of confidence once it was removed from him again just held him back from ever getting back to his absolute best. He was always a guy who seemed to thrive on confidence and when he had his tail up, and losing the Captaincy seemed to make that harder for him to thrive on.
Still a really, really good player in general, and a gun on his day though. He loved a big occasion, games like Origins, Finals, top of the table clashes etc. He had a game just before he retired where he just made life hell for JWH and the Rooster pack in general. That was the Sam we all knew and loved and that he was always capable of, but unfortunately probably didn't see regularly enough towards the back end of his career.
Still, he retired as one of the more decorated Forwards of his era and picked up more Rep jerseys and had more success in general than a lot of players that the Sydney/NSW media would probably rate higher.