“Equal exposure” doesn’t necessarily mean that an equal amount of people are actually going to click on the banners/ads. Nothing of the sort. There’s a lot more to the equation.
Before you start jumping to conclusions, you really need to take a scientific approach to tracking your traffic. Guessing and assuming will get you nowhere on this one.
Install an outbound click-tracker, or better yet, an advertising management system. A decent free one is http://www.openads.org.
Once you can actually see how many impressions-vs-clicks you’re getting for each banner of each casino, you’ll be in a better position to make an informed judgement about what is, and what is not working on your sites.
Sorry if that sounds a bit narky, but it’s just the truth of the matter 
Hope it helps.