You know that's a great question! IMO, CR gets challenging when the answer choices are NOT clear-cut. It then becomes even more important to spend as much time as you can breaking down the argument. You may make a prediction, and then find that your prediction is actually NOT related to one of the answer choices, which instead focused on a different piece of the argument. I haven't seen a really obvious difference in format between easy and hard questions. Some of them just require firmer logic and don't seem as tightly constructed as others.

For a CR challenge, you may want to check out some LSAT CR. Some of those are whoppers.