Hi gokul prasad,
Many Test Takers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so it's likely that you just have not put in enough time and effort yet. In addition, the OG books are great sources for practice questions, but they're not designed to teach you Tactics, patterns or the little 'secrets' behind the GMAT - to learn all of that, you'll need Course-oriented materials.
Given your goal score, you're going to have to make some big improvements to how you handle BOTH the Quant and Verbal sections. With your current Test Date, you have less than 2.5 months remaining, so you're going to have to do a lot of work in that time - so you might end up needing to push back your Test Date. Thankfully, you don't have to decide anything on that point just yet.
1) How many hours are you planning to study each week?
GMAT assassins aren't born, they're made,
Rich