Heath is a great tackler as long as he doesn't have to move sideways to do it.
Honestly, I can't see how you guys don't see this. He simply has tight hips, which means he can't "break down" to make a tackle. If Heath can tackle the guy right after he gets the ball, he's great at tackling. If the ball carrier has time to make a move, Heath often misses, because he can't adjust.
It's not that he needs to learn, as some are saying, it's that he lacks the physical ability to do so. You don't learn lateral agility.