Hi Flo,
Many Test Takers become too fixated on the 'implied level' of the questions that they're working on - instead of what really matters: defining WHY they're getting questions wrong and becoming more efficient at approaching the overall Exam. If you're looking to improve your existing skills (and Scores), then you don't necessarily need to be working on "harder" questions.
Before I can offer you any advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What study materials have you used so far?
3) How have you scored on EACH of your CATs (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School?
7) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich