One of my favorite Googlers, John Mueller, posted a response in a Google Webmaster Help thread that I would like to highlight.
He basically said, having a fantastic web site is great but it isn't enough for Google to confirm that your content should rank well.
From what I can tell, your site is still fairly new - with most of the content just a few months old, is that correct? In cases like that, it can take a bit of time for search engines to catch up with your content, and to learn to treat it appropriately. It's one thing to have a fantastic website, but search engines generally need a bit more to be able to confirm that, and to rank your site - your content - appropriately.
Now, since I live in the SEO world, seeing something like this makes me look at every single word and analyze it - probably when I shouldn't.
Google has always said, don't worry about all the little things - just build the most fantastic web site you can and everything else should fall into place. Somewhat contradictory. Again, John is answering a specific question so taking him within that context is important, which I am not doing.
But clearly he is saying that having a great web site is one thing. Google still needs to verify other factors, off page factors, to validate the site is truly fantastic to others.
Forum discussion at Google Webmaster Help.
Image credit to BigStockPhoto for fantastic job