This is a tough one. Many students struggle with this. (I'm included in this group!)
I have found that there is a fine line between reading too thoroughly and skimming in too cursory a fashion. I found for myself that if I practiced (this is only on practice questions, mind you) by making a short note -- 4 or 5 words -- at the end of each paragraph, and then reading back through these, I was able to identify the main idea more consistently. The purpose in the exercise isn't really the writing so much as the pause to reflect on short sections of the passage and then to aggregate these at the end of the passage. If you read through without pausing and thinking, it becomes exponentially difficult to remember discrete sections of the passage, and nearly impossible to give a main idea that is accurate.
Try this method on a few practice passages, and if you're still having trouble, email us at email@example.com
and we can try to assist you further.
(And remember, we're only writing notes on practice passages -- the idea is to get accustomed to the pausing and thinking so that you can do this on the real and practice tests without the note-jotting.)