In my experience, Clickbank products only work when you have a thin affiliate page focusing on a niche and selling the product. It does not work as much if you own a website in the general category and put up a banner promoting the clickbank product on the sidebar.
Of course it depends on what you are selling. There are quite a few Clickbank products that can work even on a general category website. But what I've mentioned above is my experience with the majority of them.