I am a left handed pitcher and throw from a low three-quarter slot and get good run on my fastball. However, if I try to spot up a fastball to have it run to the inside corner it turns into the straightest pitch ever… It either results in a ball or a HBP. Is this a mechanical fault, or is it just much harder to make a ball move to the inside corner rather than from the middle to the outside corner?
Tough to say. Could be a mechanical issue. Would need to see you pitch. Would want to look at things like posture and balance, starting position on rubber, stride direction, etc.
It could be the angle to home plate makes the run less effective. Try moving to the right side of the rubber is the only thing that comes to mind