Review markup is only useful if it is error-free and contains all the necessary information.
You can use the Google Structured Data Testing Tool to check your markup for syntax errors and missing fields.
For administrators, to simplify the process of testing your markup, add
?strong-debug to any URL with review markup.
This will show a button named "validate markup". Click that to send the markup to the Structured Data Testing Tool.
This also works for the aggregate rating shortcode.
Please validate your markup. If you do not see results like this, there may be a conflict between this add-on and another plugin or your theme.
More examples available in the Review Markup demo.