I don't see why you need that artificial grouping dimension and one expression. Why not simply 12 expressions? You could put text boxes above (or on top of) the pivot headers if you want some kind of 'grouped' layout.
Actually, a straight table might work as well or better as a pivot table if you do that.
Without having a good look at your data model and the expressions you use it's very hard to give advice. It would help if you could post a small qlikview document that illustrates the problem.