Well..Yes...questions in GMAT prep are a bit more difficult than OG. And unfortunately so are the questions in the real GMAT.

Are you simply doing practice questions or are you actually learning how to attack the questions through proper technique and filtering mechanisms? Without learning the strategies necessary and being extremely familiar with most of the error types from the most basic Subject Verb Agreement questions to higher end sentence construction questions, really going up a notch is much harder. You have to be able to recognize potential pitfalls and ask the questions necessary for those 'red flags'. In other words your approach has to become extremely systematic. But again, I am not sure how you have been practicing apart from what you briefly mention.

saurabhmahajan wrote:Friends,

I am facing problem in solving SC questions in GMAT Prep software. My accuracy level when solving SC questions on this forum or from OG has been good, say 8 out 10 i get correct. But when i take GMAT Prep test it decreses to as low as 5 out 10. So just out of curiosity, is there diff between OG, Forum questions and GMAT Prep questions ?

Please clarify my doubt.

It looks like there is a disconnect between how well you think you know SC and how well you actually know it (which is what you see when you attempt GMAT Prep. I'd suggest figure out where (areas) you are getting questions wrong and then revisit those concepts. When I prepared for the GMAT I prepared for SC using e-GMAT .
I personally haven't studied SC from MGMAT, but I have heard MGMAT SC is a good one too.