same thing happened to me. Got a 640, went into the test feeling unprepared even though I took a crash course and studied for about 2 months. Then retook it a month later, studying like mad, and got a 600.
I attribute my lower score to mental exhaustion. The gmat is a test you can study for, but what it boils down to is being fresh for the 3.5 hour test. I think that studying too hard, at least for me, hurt my score. I mean you don't have to be a math whiz to score over 44 on the quant and if you post a decent verbal you are over 700. Doing question after question is good practice, but in the end I think I did too many. Another big thing for me the second time around was that I kinda raced through the Q, trying to get to every question. I must have made a lot of dumb errors which cost me a 9 point drop in Q, 44 to 35.
I plan on taking it again march 1st, with some studying to refresh, and take my time through the quant.
My goal is as close to 700 as I can be.