GMAT Strategy

This is the second time i am giving GMAT. Had a 680 earlier. I am looking at at least 720 this time. I am very good at quant but verbal, specifically SC is the problem area. When i gave the exam first time i dint study MGMAT SC. This book helped me a lot this time. When i sit and only solve SC i do very well. My hit rate is around 85% to 90%. But in the practice tests it is as low as 50 to 60%. My issue is i cant spot the error immediately in the exam. Please help