Zineb from Google posted on Twitter in French that Google doesn't want to clutter the search results with too many rich snippets. She said, "enfin, nous essayons de ne pas trop "encombrer" les SERPs avec trop de sites avec RS. Du coup, vos critères sont à évaluer." That translates to "Finally, we try to not "clutter" the SERPs with too many sites with RS. Suddenly, your criteria are to evaluate."
Google has done several rich snippet reductions in the search results in the past. Of course Google only wants to show rich snippets where it makes sense and helps the searcher.
To show 10 out of 10 search results with rich snippets may sometimes make sense but often not.
So that is why it is hard for a site to always show rich snippets in the search results. It is not just site specific, but also query dependent and other criteria.
As you can see, Google is very conscious about the rich snippets and as they add more support for it, they don't want to overwhelm the searcher with too much eye candy to look at.
Forum discussion at Twitter.
