Too many times over the years have I seen cases of too much physical instruction. While the actual instruction might be outstanding, the problem isn't always physical. In fact, most of the time it is mental. When a hitter has learned how to hit by a knowledgable instructor and has had success, and then all of a sudden he quits hitting, it's not very often technique, it's usually his mentality.
At one time or another we have all experienced this negative mentality. We tinker with the swing, we change bats, we use a different helmet, or even eat something different before the games. Sooner or later, provided you know how to hit, you will hit. Too much of the other stuff just gets in the way and clutters your mind. The more clutter, the bigger the problem becomes.
Now obviously if there are major mechanical flaws that is a different story. Maybe there is even a small adjustment that needs to be made. So a good hitting instructor can be very valuable. But one thing to always keep in mind, is the mind! It is the most powerful thing any hitter has ever owned. Also keep in mind there is natural ability involved in all baseball skills. Because of that, I've always been against the cookie cutter approach.