Mike Tomlin isn't THE problem, but he is A problem.
He can't manage the locker room, and has never been able to. He has loyalty to some bad defensive coaches (Particularly the embarrassment that is Joey Porter), and he just lets the players run the team. I have a hard time believing there is any player on the team's roster that legitimately respects Tomlin as a coach.
They may respect his position, they may respect him as a person, but I can't imagine that guys in the locker room observe the situations of AB/Bell/Wallace/Bryant/Harrison and have respect for him as a coach the way that players respect a coach like Belichick.