Reading Comprehension

when i write the whole exam completely under test condition i am getting 48-Q ,27-V.After i review i can answer all verbal correctly,i don't know why i make silly mistakes.But if i do just the verbal session i am scoring 35-V in mgmat cat.I don't know what makes such a big difference.I don't know what else to do,i know all grammar,good reasoning,RC is my weakness but i started reading lot of books.But what makes such a big difference and how can i rectify it?please help...