If you find it hard to understand a passage I think it's a good idea to take the very passage you have trouble on and dissect it. There's a great technique called bracketing, where you look at the questions first, and then put a bracket around the part of the passage that the question relates to. That way instead of trying to take in the entire passage at one time, you can focus on the specific part that's most important to you (the part that'll give you the answer).
Usually in the critical reading section they've set a balance: the harder the passage is to understand, the easier the questions, and the easier the passage, the harder the questions. This way bracketing really comes in handy, because the easier questions directly tell you what part of the passage they're looking for.
If they're giving you tone or author's purpose questions, then you're in a little more trouble. But I still recommend breaking up the passage into more manageable parts. Also, I find that as you answer more questions the passage, the clearer it becomes. Look back at the passage, and figure out what parts are the hardest.
If you would like to point me out to a passage (if it's in the blue book) or give me a sample of what's giving you the trouble (without breaking copyright laws), I would be more than happy to help you out.