Can you improve? Yes. You just have to make sure you're doing the right things in the right way - and you have to understand WHY you have the setbacks that you have.
Why did your score drop so much on GMATPrep? Did both sections drop, or was it mostly only one? Did you have any timing problems? That's one very common cause of score drops. Did you alter the testing conditions somehow? Eg, did you not do the essays on the "higher score" test but do the essays on the "lower score" test? Did you burn yourself out right before taking the lower test, or did you take it too early in the morning or too late at night or anything like that?
You can use this article to help you figure out why your score dropped so much:
http://www.manhattangmat.com/blog/index ... ent-wrong/Then, come back here and tell us what you discovered. Once we know WHY this happened, we can figure out what to do about it.
Also, if you'd like to get more specific advice on content areas and what to do, use the below article to analyze a recent MGMAT CAT:
http://www.manhattangmat.com/blog/index ... ice-tests/Note: if you discover through your first analysis (about the score drop) that the 640 test was inflated for some reason - you skipped the essays, and you paused the timer to give yourself more time or something like that - then you may want to take another CAT under 100% official conditions so that you're certain that you're getting good data.
Then you can come back here again and tell us what you found out in your second analysis (do these separately - don't put in one big post or it'll get too confusing). And we'll figure out where to go from there! :)