Google changed star ratings and how difficult it is to show them. You can add them via Rich Snippets. Your site may not be the kind of site Google will allow to have star ratings. You can do a Google search for more information on star ratings, what you need to include your site and the type of site you need to have to show star ratings. Just know as an FYI that my Google sources have told us that eventually Google will likely remove most if not all types of star ratings on SERPs at some point in the future.