I think this is sort of common when trying to get approved for Adsense from HP. It's probably just because of that, you have too little of content, even though they recommend 10 hubs before you apply, with Adsense you want to be sure you will get approved, so apply when you have 20.
The way I got approved for Adsense was by starting a blog at blogger, this seemed to be the best way as Blogger is owned by Google. After about 18 posts I was approved, and then I just added it to this site and Webanswers.
Webanswers is another place where you can make money and get approved fro Adsense. You have to answer 50 quality answers before you apply, but like I said, should go beyond what they recommend.
Here is an hub about Blogger:
And an article on my blog about Webanswers, which is a great earner to let you know: