He never needed a fixing when he came up though. His swing was pretty much reinvented in the minors and their attempts to fix the part of the swing that wasn't the problem is was fueled more struggles. Belt didn't want to change because he knew that wasn't he issue. His hands were not the problem in the swing, in the minors they fixed that along with his overall stance + hips/weight movement.
They kept moving his hands around at the major league level but never fixed the hips moving too far forward and how his head followed. They also never addressed the fact that his weight shift was completely unbalanced in his swing. All of that was Belt's fault but the fact that they changed a near perfect swing as soon as he got to the majors was a big part in his collapse.
Basically, they should have never messed with his swing. When your weight and hips start to betray your swing, you can't just have the player move his hands. That doesn't address any of the problems. Once they started addressing the real problems in ST before last season, you saw the results. But it's up to him to keep that going which is proving to be another problem unfortunately