One thing I have noticed is that the questions on tests made by test prep companies can tend to be subtly different from those on the official test. The questions on official tests tend to be tricky in their own way, and test you in a way that is all their own.
So, it's possible that one thing that is going on here is that you didn't exactly prepare for the GMAT, you prepared for a MGMAT CAT, which is close, but not exactly the same thing.
So maybe the next thing you should do is take an official practice CAT to get a better feel for official GMAT questions and to get a sense of what you need to do to raise your score.
There are four official practice CATs available. Two of them are included in the GMAT Prep software available at MBA.com and the other two are included in PowerPrep, which is the predecessor to GMAT Prep and which can be downloaded from the Resource Links page of Beat The GMAT.
I can tell you that my scores on GMAT Prep and PowerPrep tests were within 20 points of my actual score, and I definitely noticed the difference between them and practice CATs made by test prep companies.
Another thing you could do, if you have not already, is make your study test centered rather than concepts centered. What I mean by this is take an official practice CAT, or two, and assess from that what you can change to raise your score. You already have exposure to most of the concepts that one might apply on test day. Now focus on developing skill in applying those concepts to getting correct answers.
Now check this out. In a way, this is a test of how many correct answers you can get in a row. In other words, one can get questions right, but, if they are not in a row, then that doesn't raise your score. On practice tests I have seen a person get right significantly more questions than I did, and get a score 150 points lower. So that could be part of what happened in your case. In order to get the higher points awarded to harder questions you need to keep getting questions right as they get more challenging . So by preparing for MGMAT CATs rather than the actual test, you might have just been enough off your game to not get those more challenging questions right.
In addition to using the official CATs and your own assessment of things, you could use other resources such as this forum and other companies' materials. Each person or company has a different take on what it takes to score high on the test and by amalgamating ideas from all of it, you can optimize your preparation.
Finally, it seems that one thing I have been seeing is that often people prepare for this as if they were preparing for a class related test. They study material and learn concepts and then see how they do. The thing is this is not really a test of concepts and rules. While obviously they are useful, the concepts and rules are just building blocks for tricky questions, and in some cases even, questions one runs into on the actual test can be rather different from anything one has seen in preparation. So, just to be clear, what you really need to be doing is getting better at answering tricky questions. This is about you and your skills, more than concepts or rules. So to raise your score focus on that.
