It isn't hard to get your hubs featured, but it is a good achievement to have all of your hubs featured.
I have about 35 hubs, and all are featured. If a hub is not featured, it means you should either revise it, or simply delete it because it really isn't going to make you any money. (I would't suggest deleting any hubs, much better to revise!)
As long as you write good quality information that people want to read, you should have no problem getting every hub you write featured! Keep up the good work!