A bottleneck faced in the Verbal part - Please help

I have targeted for a PhD in business, which means I should obtain at least 700 for my overall GMAT score.

In February 2015, I had my first GMAT attempt in order to fulfill the condition required for a Master degree position.

It was a pretty low score requirement. Besides, I was occupied by my full-time job. So I did not spend too much time on practice, apart from getting myself familiar with the examination format.

Eventually I scored 590 - V24, Q47, AWA5.0, IR4. That means my mathematics is not a major problem though I can be better.

And now I am able to spend more time to practice verbal. However, I have been trying many different sets of exercises, including the Kaplan Verbal Book and the official guide. I can only get, on average, 7 out of 10 questions correct for all types of questions in the verbal part. I am quite upset, since it would be quite a long distance for me to reach the desired score.

I admit that I have not tried any sort of free test which will predict a score, because I think I am not ready in the light of the above situation.

I am to take the next GMAT in Mid-September. Please kindly advise how to break the bottleneck. Thanks a lot...