It's kind of the two-edged sword with his natural mechanics. His ability to effortlessly twist and change direction while throwing off his backfoot to the opposite hash undermines his downfield throws, because he does so reflexively. I.E., if and/or when he learns to consistently step into his downfield throws, he'll achieve next level performance. He's just inconsistent with it now, and I believe it has to do with improving his poise so he can concentrate. In the first half, as in other games, he was antsy in the pocket. The OL could have done better to help with that early on too. Under duress, he reverts to his reflexive throwing mechanics. When he's able to maintain concentration, he can step into the throw and nail it. That's my opinion. Other thought is he also waits too long to make many of his downfield throws. That's another two-edged sword I believe is a result of his risk aversiveness (good thing trying to avoid throwing INTs) but often prevents hitting the open deep receiver in stride resulting in DBs catching back up with the receiver.
Heacock also did well dialing up the best scheme to make Quinn uncomfortable with what he was seeing downfield. Not surprising as he is a guru of it and had an entire season's worth of film to prep with.