GRE Long RC

I completed 14 tests of the Big Book and got an average of 3/4 in short RCs. But in Long RC i struggle to find the main idea and in average i get 3-4/7 right(In 10-12 mins).

I went through all of greg reading videos and struggle a lot in Long RC. Can anyone suggest a way to improve it.

get good at finding structure and main idea . For this you really need to do a good amount of untimed and timed practice .