Hi erikvm,
The problem you're describing actually happens to many Test Takers who use a "book heavy" study approach. Even the best books are limited in what they can teach you; they also can't force you to approach questions in a certain way and their explanations are often one-sided.
"Your way" of approaching the Quant section has you stuck at this current scoring level. To make a big improvement, you'll have to change your tactics. This will likely mean investing in some new Computer-based resources and learning (and practicing) new tactics.
I'd like to know a bit more about your last CAT:
1) What were your Scaled Scores on the Quant and Verbal sections?
2) Did you take the ENTIRE CAT (including the Essay and IR sections)?
3) Did you have to rush to finish the Quant section? If so, then how many questions did you guess on at the end just to finish?
After reviewing the section, how many questions did you get wrong....
1) Because of a silly/little mistake?
2) Because there was some math that you just didn't know how to do?
3) Because the question was too hard?
Thankfully, the GMAT is a predictable, standardized Test, so you CAN train to score at a higher level.
GMAT assassins aren't born, they're made,
Rich