Hi Deepak
Sorry to hear about your score, but I guess it happens. Your verbal seems weak. You will need to work on that aspect. Mastering Sentence Correction is the way to go. It's very much like maths and there are a fixed set of rules that you need to look out for. Follow the Manhattan Sentence Correction Guide. It gives clarity on the many expressions that we misuse in daily life.
Also, you mentioned that you never got below 30 in the prep tests. Well, how many questions were repeated from the OG/Review guides in those tests? That should give you an indication. If no questions were repeated, then you're fine. However, you need to work on your verbal. Your maths is well above average, so I suggest you not look at Maths at all till the next test. Do only verbal, verbal and more verbal.