But the two main reasons that I found for Tannehill not throwing a proper deep ball was first he did not always have great protection, and second his footwork and lower body technique were not always what they should have been.
Tannehill has demonstrated throughout his career he has a Howitzer Cannon for an arm and I do not think anyone who knows how to evaluate a quarterback, even just a little, can argue that. The arm strength can primarily help with intermediate and sideline passes.
But if you have poor footwork and you do not transfer your body weight properly then the strongest arm cannot be successful throwing the deep ball.