Unfortunately there is still no statement from the big search engines if, how and when Rich Snippets show up on their search results. This makes it super difficult to give you an exact answer. Here are some notes that could help:
- Snippets don’t show up after you’ve hit the save-button in the plugin. Search engines must crawl your site after you’ve added or changed your snippets on your website. If you add new posts on a daily basis the crawler will recognize changes faster then on other sites.
- Verify that Google Rich Snippets Test tool does not show any errors on your sites where you’ve added rich snippets. If there are errors, please check if your snippet is correct.
- Nowadays search engines use artificial intelligence to personalize search results. So it can be that you do not see your snippets in search results whereas other users see it. Try to clear your browsers internal caches and cookies, log out of all services and try a new search to check if your snippet shows up.
- If your snippet still does not show up, there might be other problems with your SEO quality. You should read trough Googles Webmaster Guidelines to get an idea of what might be wrong.
- Here is a very good blog post on SearchEngineJournal that explains why your rich snippets may not show up.