His problem is the same as it was when he was in the minors. As the scouts said about him then, he doesn't have any pitches that are going to get left handers out on a consistent basis. Unless and until he develops a pitch that can get lefties out he is never going to be more than he is now, which is a back end of the rotation starter at best.
Which is why I'm absolutely shocked He hasn't developed a better feel for his change up. Would help neutralize Lefties and would be a nice with the Mid 90s fastball and sinker