It looks as though your hubs aren't doing well enough yet. Work on fixing them up so that they are evergreen hubs on attractive subjects. With over 20 published hubs in a year, I would look to the content and quality of your already-published hubs. Fix them up so that they are the Best on the topic that you're writing about. Interact more on other people's pages to gain a larger following. It's sort of a traffic and numbers game for AdSense. If they can't make any money on your content, they won't allow you to join until you can do so. Good luck!