Hi adi,
First off, a 720/Q50 is an outstanding score (it's above the 90th percentile overall), so you should apply to any Business School that interests you. As such, a retest might not be necessary. It's important to remember that highly-competitive Schools require a strong OVERALL application, so raising your GMAT score a few points might not be as meaningful as you think it is. Since you're considering Top 10 Schools, you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
ask-admission-consultants-124/To answer your questions:
1) If you can't figure out why you're hitting a Q50 instead of a Q51, then the reason is probably DS (you're missing a couple of 'gettable' DS questions because you're not considering certain possibilities).
2) The Verbal section of the GMAT really IS as predictable as the Quant section is. If you're 'stuck' in the mid-30s in Verbal, then you're probably not using the proper Tactics for that section. You might also have developed some 'bad habits' during your studies that are limiting how high you can score in this section.
Do you know the exact Schools that you plan to apply to?
When are you planning to apply?
GMAT assassins aren't born, they're made,
Rich