Google is now showing the Review Rich Results only for a clear set of schema types for review snippets, reviews contain name property within the markup, & non-self-serving reviews.
Following are the types of schemas which are allowed to show the reviews:-
Self Serving Reviews are Not Allowed
Reviews noticed as self-serving are not allowed. “When a review about entity A is placed on the website of entity A – either directly in their markup or via an embedded 3rd party widget”. Google Said.
Name Property is Required
Google said- when you write a review about something the name property to the markup should be added to it to certify for this review markup. The name property is now mandatory, so do remember to mention the name of the item being reviewed.
What are Review Rich Results?
Below is a screenshot attached of review rich result: