Hi MulanQ,
Since you're posting here, I think it's safe to say that you're looking for a way to improve your GMAT scores. You haven't provided much information though and saying that we should "trust" that a GMAT Course won't help you sounds "silly", since they tend to help 1000s of Test Takers every year. To be fair, you MAY be an exception, but we don't have any proof of that.
If you can provide some additional information, then I should be able to provide the perspective that you're looking for:
1) How long did you study before each of your GMATs?
2) What resources did you use (books, Courses, etc.)?
3) What were your Official Scores (including your Quant and Verbal Scaled Scores) on each of your GMATs?
4) Do you have a pacing problem on any section of the GMAT? Do you ever finish early? Do you have to rush through a section and guess on questions at the end just to finish (and/or leave questions unanswered)?
The GMAT is a predictable, standardized Test, so you CAN train to score at a high level. You just haven't done so yet.
GMAT assassins aren't born, they're made,
Rich