If you're have some problems with locating your pitches, perhaps, indirectly your backstop is influencing you in subtle ways.
For example. A very low glove position can have you driving your pitch into the ground. Sometimes without realizing it, your backstop can position his mitt so low that you inadvertently target that mitt- but, because you're over 60 feet away, the downward trajectory is just too "downward."
Another example. Let's say you pitch is to be outside. If your backstop positions himself too outside, you're targeting something that is off the plate without realizing it.
So the next time you're just not cutting it for whatever reason(s), think about your backstop's influences behind the plate.
Now I'm not suggesting a blame game here, nor am I suggesting how to coach backstops. Coaching backstops I am NOT qualified to do. Just be mindful of the subtleties that can influence you, pitch after pitch.