If timing is not a problem, then I would suggest the following.
Ask yourself what type of questions are you getting wrong? Is it the same type of questions for every passage? (I believe this is most important in diagnosing your problem)
Are you missing questions related to identifying the theme/missing the author's point?
Most of the time, the answer lies in the tone of the question.
Can you elimate two responses out of the 4 possible choices?
In order to raise that score you'll need to practice daily. Get your hands on as many tests as you possibly can. I'll recommend also buying the examkrackers verbal/mathmatical reasoning book. It has techniques that aids in raising the verbal if timing is not an issue for you.