There are many possibilities as to why you are struggling, but don't give up hope! I would definitely say that 8 hours of studying a week is not enough. I would put more time in. Typically I would recommend atleast 15 hours a week if you can spare the time. Make the GMAT your part time job.
Also, it sounds like you are concentrating heavily on practice problems, but still not having success. This shows me that you may still not be grasping the concepts, and have gaps in your knowledge. I had a similiar issue when I first started studying, and ended up going back to the basics. I went through the KAPLAN
math refresher, as well as the similiar product that Manhattan provides online once you've purchased any of their books. Going through easier concepts and easier problems helps build a strong foundation. You can't just jump into 650-700-750 level questions without a solid foundation in either section. I would go back, work on that foundation, work on your strategies (back solving, picking numbers, taking notes etc) and recognize when those strategies will come in handy. You are close, but just remember that just slamming out practice problems for 8 hours a week isn't going to do it. Each time you start a problem and hit a wall, that means your missing something. Figure out what it is, commit it to memory. You're competing against many talented individuals. I personally studied for about 20-25 hours a week for 10 weeks for my score, because I started at a 570 and new I wanted to crack 700. Nothing else would get in my way. I work full time and put in 2-3hrs on week nights and 5-8 hours each day on the weekends.
You'll get there! Just don't give up, and keep your nose in the books. Low 600s is already a strong score, and going back and building that foundation could easily get you that 50-100pt increase your looking for.
PS there are many extremely challenging problems near the end of each section in the OG. YOU DO NOT HAVE TO GET THESE RIGHT TO SCORE A 700. Many people concentrate on getting the hardest problems right, when the GMAT just wants you to get everything YOU can get right. If you get all the 550-600-650 level questions right, guess what??? you just got a 700+. You still could miss all the tougher problems and be where you want to be. Make sure not to get caught up on the toughest of problems, and work on the problems that you CAN get right in two minutes, and once you figure out which problems are the toughest you can ditch those, and give yourself more time for the problems you can get right. The way the GMAT works, you need to get the easiest questions right to even get asked the tougher questions. Stringing together the easier problems will really boost your score. You have to know how to solve these, and in a timely fashion. Even right b before my test, those back of the book questions were still blowing my mind. Good luck!
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