Rich Snippet - Schema Markup - Google Not Displaying Summary Review Stars in Search Result

Google may or may not show the review stars in their search result even if you have the schema markup (rich snippet) setup properly. First, you should use the Google Structured Data test tool to make sure there are no errors...

https://search.google.com/structured-data/testing-tool

If that checks out, then basically it is up to Google to add the stars. Here is a great blog post about why they may not and some tips that might help.

https://whitespark.ca/blog/how-to-use-aggregate-review-schema-to-get-stars-in-the-serps/