Maybe you lack focus OR your court positioning/movement are bad?
you're thinking too much of other stuff and not looking at the ball: will i make the next point, will this be a winner, can i keep this up? etc...
positioning yourself, still looking at the ball, bouncing/running/sidestepping to get in position, still looking at the ball, swinging through the contact zone, still looking at the ball, and so on...
Court positioning / Movement:
I recall you questioning if there is strategy in tennis. This is where it comes in. You're hitting on the run poorly because you are doing one of two things wrong: A)
You're not constructing your points well enough (directionals, shot selection, etc...) and/or B)
You're not getting into the right position to cut off tough angles and benefit from your previous shot (i.e. approaching after good shots, getting back to the court after being pulled out wide, aide and split stepping appropriately, etc...); Fitness matters in this respect as well.
Or maybe your stick is too heavy for you or not headlight enough?