Nagy's a great coach, he's just hamstrung because Trubisky isn't good enough to execute his genius playbook. All Nagy has left in his playbook are bad plays that get blown up, the genius ones can't be called because of how bad Trubisky is!
Trubisky could be a great QB, he's just hamstrung by Nagy! Nagy sucks as an OC, he can't get a running game going, he calls stupid plays, the OL can't block, and because of all these things, the defense just keys in on Trubisky and shuts him down.
Here's a thought: They are both shit. Get rid of both of them.