I'd like to add my 2 cents
Every shot, every time we set up and stroke requires precision.
The reason most of us are not better is because there are tiny flaws in our stance, delivery, timing, eye movements, grip, consistency, etc, etc. Not to mention, focus, concentration, confidence, clear mental picture of the entire shot and the sounds of the shot, including the path of the CB, etc, etc.
We get away with a lot of shots without sometimes even realizing something was wrong in our execution because the shot may have been more forgiving than the long off the rail variety.
Shooting off the rail on a long shot is simply another shot. However, it is one of those shots that magnifies the need for preciseness of everything I mentioned and probably many more things.
Practicing of course is necessary, the catch 22 is continued practice of any one of the above things incorrectly, screws up the rest of them.
The correct combination is different for each and every one of us. Finding the correct combination is like assembling a complicated puzzle that includes a ton of pieces from puzzles that don't even fit in the one we are trying to put together.
Falling into one particular facet correctly turns the light bulb on, but ... initially it requires us to consciously remember our important find.
Unfortunately, this detracts from our focus, concentration and even some of our other mechanical execution.
So, what should we do? Go Bowling. No .. .just kidding
Don't keep practicing the same thing over and over that isn't working. Keep an open mind, remember we all react differently to different techniques and finding what we need to do is a very delicate and complicated task.
The long shot just magnifies what we are already doing wrong.
Practice feeling comfortable shooting off the rail across the table rather than up and down its length. I know you probably can do this with no problem, the thing is to become sensitive to the tiny uncomfortable feeling you experience while doing it and try to change things to eliminate them. Be super aware of anything that doesn't feel perfect.
For me, I shoot these pretty good, but I enjoy straight pool and that's often the first shot in the game. I like to envision 2 imaginary laser lines. One from CB to OB and one from OB to pocket.
Once set, I keep my eye on the OB. The long distance between the two balls will really screw my focus up if I shift back and forth. Then I cover the imaginary line from the CB to the OB with the shaft of my cue as I stroke through the ball.
I don't wanna get into what part of the shaft I aim with, cause .... ya know ...