Hey there,
To begin with, you should note that the GMATPrep will adjust for question level. In my case, I regularly found the average difficulty on the practice tests to be higher than in the OG. I figured this is because of the adaptive software adjusting the level to give me questions consistently challenging to me, rather than the OG which would essentially contain a random distribution of difficulty levels.
I struggled with CR and RC briefly because these are very GMAT specific topics with GMAT specific ways of understanding them. I would advise against referring to the Manhattan Strategy Guides for these topics because, awesome as the Manhattan tests are, the Strategy Guides overengineer the approaches to tackle CR and RC questions. Have a look at the Kaplan material, if you can get your hands on it. I read through it once and their points stuck with me. They're concise and effective. Follow what they say to the T and see if that boosts your score a bit. The answer options tend to be formulaic. If you figure out how to stay within scope and identify patterns in how they choose their answer options, you'll game the system easily. Therefore, read the Kaplan points, implement them verbatim and practice 150-200 questions and you should see some result. Thereafter, rinse, repeat.
For SC, I'm afraid I can't offer much useful guidance without knowing the level that your English is at. Being from India, I grew up speaking English as my first language and to me SC questions were rather intuitive and less application of rules. However, if you do need some help, the Kaplan material and the official GMAT Verbal Review have useful material you could peruse.
Hope that helps somewhat!
Cheers and good luck!
