I think he just doesn't have the feet to match up athletically with the level of player that he's lined up across from. I'm sure he specifically trained and ran that combine drill 5000 times before he did it for the stopwatches, but repeatedly practicing the same rote series of maneuvers is a far cry from having to be able to move your feet and adjust on the fly when you have no idea what's coming from your opponent. He just can't keep up with those faster edge players when he's executing full speed in real time.
That boy is massive and country strong; I don't think leverage is as much an issue for him as he might be for other players. When he gets his hands on people, they're in trouble, so even if he gets jacked up occasionally by a lower man he has enough power that he should still be able to control him. I'd be more worried about hiding his feet than his knees.