Hi Kanokporn,
GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Your 2 Official Scores are in 'range' of one another - and show that you essentially performed the same each time (about 630 +/- a few points). To consistently score at a significantly higher level, you're going to have to make some changes to how you handle BOTH the Quant and Verbal sections.
Before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long did you study for each attempt at the GMAT?
2) Did you use any other study materials besides the books that you mentioned?
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 apply to Business School?
6) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich