A recent article on Ad Age reminded us that when evaluating the performance of online display ads, the art, copy, size and position on the page aren't all that matter.
The article points out the importance of environment. In other words, what is the visitor doing on the page when they see your ad.
For example, if you run the same ad in an email inbox that you run in a side bar on a blog, you shouldn't expect the same results. People in their inbox reading/writing email are a lot less likely to allow distractions than people reading a blog.
If you are running a Google Content Network campaign, you should take a closer look at how you are testing creative. Try some managed placements to help eliminate the environment variable.
Check out the article: Location Matters: How Ad Environments Affect Performance - Advertising Age - DigitalNext