I went back and watched highlights from 2014 and I think I have the answer....
Hand talent.
Michael Thomas, Devin Smith, Dontre Wilson, Heurman, Vannett.... They all had sticky hands. JT could trust the shit out of them that they were going to catch the ball when he put it out there for them. I don't see that trust developing with anyone and he is being careful where he places the ball. He had that trust with Brown last year, but Brown wasnt a downfield threat. I think the silly "6 starters" rotation is hindering that. He needs reps with his core so they can trust each other and develop.