Well first off, I got 640 in the actual GMAT on my second attempt, not gmatprep...
First find out which type of questions you've got wrong. Word translations or number properties or geometry etc. Try and practice a few difficult questions of those types.
For RC, have u tried all the questions from OG? In "what is the purpose/point" of the passage, the correct answer is always the one that describes the whole passage in 2 sentences. U're saying u can comprehend the passage. After comprehending it, imagine u have to sms/text the summary of the passage in 3 lines to a high school student! u'll find that answer clicks with the option. U'll get a hold of this only after practice.