Why is Google not showing rich rating snippets for my website?

GD Rating System is adding a fully functional and standardized rich snippet for rating or review, and you can test all your posts to confirm that the rating snippet is correct. But, there is no way to influence Google to show the snippet inside search results. There is no way to know how, why and when Google displays rich snippets and even big websites like IMDb or Amazon very often don’t have the rich snippets displayed.

Check out this user guide about setting up the rating rich snippets.