Knowing you need to work on a particular shot isn't so much of a mystery as knowing how you should work on the shot. Everyone agrees that work is needed -- that's non-debatable -- but the "how" seems to be a matter of varying opinion, despite the abundance of research available when it comes to skill/talent and habit development.
The old-school "work" method would have you hitting several hundred balls per day in order to get a particular shot under your belt, practicing for multiple hours daily, the HAMB method. Many pros and instructors have learned this way, so they will surely stand by that approach. But the HAMB method has taken a back seat to more effective learning methods, like replacing long dreaded practice sessions with much shorter practice sessions spread throughout the day, where quality of practice has proven more effective than quantity of practice.
Pay attention to how you miss. If you notice a pattern of overcutting or undercutting the longer rail shots, then it's likely (as Fran or someone already mentioned) that you aren't seeing/aligning the shot correctly. You are in the habit of aligning yourself out of tune with this particular shot, for whatever reason.
The cure is to consciously/purposely fix the alignment, either by approaching the shot slightly thinner or slightly thicker than you think (based on how you typically find yourself missing the shot). Ftom this mindful adjustment you need to shoot 20 or 30 shots and then take a break and think about what you did and what the difference was. You might find that the adjusted alignment clicked/resonated quickly. Or it may feel awkward. The key is to keep doing this, hitting a few shots with 100% mindfulness in approaching and lining up for the shot, settting a goal to pocket 5 in a row, then taking a break, then doing it again a few hours later. When 5 becomes easy set a goal for 7 or 8 and so on.
Within a week or so you'll begin to notice that you aren't consciously trying to line up a little thinner or thicker anymore -- you'll be doing it automatically without even thinking about.
