The first thing Lee needs to do is jump. He does not jump to contest shots. Secondly, numerous times on switches he doesn't come out to contest shots.
Actually, the number one rule is NOT to jump to contest shots. It puts you in the position to be blown by and also to get a stupid foul. When you're man is shooting, you go at him hard, with your hand up and in his face, but ready for him to drive or pass. If you happen to block his shot, cool, but I'm sure 95% of blocked shots come in the paint.
When you're in the paint, it's all about timing and reflexes and THAT is a skill. It's a skill that Marcus Camby has mastered, but some people will just never have it. David Lee will probably never be a shotblocker. It's not a part of his basketball "Instincts" nor should we expect it of him.
He does need to rotate quicker on help defense though and he needs to come out stronger on shooters and he needs to. He needs to have a clue what he's doing on the help too.
I'm not that concerned about his man to man. He does his best in that area.
“Whenever I’m about to do something, I think ‘Would an idiot do that?’ and if they would, I do NOT do that thing.”- Dwight Schrute