Paul Perillo from Patriots website was asked about McCourty and this was his reply
"McCourty is struggling, there’s no question about it. He’s been beaten often on deep balls down the sideline as well as crossing routes over the middle. His interference penalties have come on plays in which he’s been badly beaten and racing to catch up. That’s why he hasn’t been looking back for the ball. So, until he can figure out how to keep his man in front of him I wouldn’t expect to see his head looking back for the ball. It’s hard to do that when you are chasing."
That is a good point. If you're trying to catch up then you're not in position to turn your head.