Hi,
I am no expert but I thought of trying to help you out.
As everyone would agree MGMAT SC is the best book on SC by far but if this is not helping it might make sense that you try and change something in your study pattern. Maybe use some expert help. I would also maybe suggest some prep courses to help you help out on your areas of struggle. MasterGMAT and knewton are two very good options if you decide to go that way.
I can talk about knewton because I am using it presently. I would seriously recommend it for ne1 trying to improve their verbal. They cover all the bases and really help. And on top of that they have a 50 point guarantee. This means you will get your money back if you don't score 700 on your next attempt.
So do think about these options and try and do something different in your prep. It is also necessary in SC to know why the other options are incorrect. So spend time on every question, just solving and moving on will not help. Also, make a error log and revisit the questions till you know all the rules tested. Its what they say, quality over quality.
I hope my post helps.
All the best.
Cheers!!
Please press "thanks" if you think my post has helped you.. Cheers!!