Rich snippets and review snippets
Rich Snippets (also known as Rich Results) is a type of web markup that allows Google to better understand what information is found on a webpage. They are small blocks of text beneath a search result in Google.
Review Snippets is a part of rich snippets. It shows the stars and aggregate ratings for a product.
Google doesn't accept rich snippets for the homepage, Reviews Page, and collection page.
They state that "rich snippet annotations should be placed on leaf pages" and they should only “provide review information about a specific item, not about a category or a list of items."
Rich snippets warnings and errors
Google Search Console
Google Search Console helps to inspect a specific URL of the website, but it only identifies warnings, not errors.
Rich Results Test
This tool https://search.google.com/test/rich-results lets you individually test your website pages and identify errors (if any). Please check with the products that have reviews only.
However, this tool does not update the changes immediately (even when you have refreshed). Instead, try to:
Scan another product page URL. This will force the tool to update the results.
Or copy-paste the Page source into the code section, instead of the URL section.
Identify structured data formats
Microdata | JSON-LD (Recommended) |
It uses HTML tags to name the properties you want to expose as structured data. When you click on Type: product in the product tab in Rich Results test, you'll see this:  | A JavaScript notation embedded in a <script> tag in the page head or body. When you click on Type: product in the product tab in Rich Results test, you'll see this: 
|