A few things that came to mind:

1. Are you redoing the same books or practice tests over and over? Even though you might be getting some new questions, you might also be getting most of the questions in the books/tests right on the second (or third) try because you already remember them.

2. Remember that the GMAT is adaptive - are you only getting the easier to medium level of difficulty questions right? That might account for why you're not scoring as high as you'd like.

3. Try to practice pacing when you're taking your practice tests or even when you're just working on problems - it could be just that you're not used to testing under the constraints of time.

If you could post a few more details, such as the breakdowns of your scores and etc, etc, that would be of more help.