Review star rich snippets are a rather poorly understood feature of Google’s search results, especially for local businesses. There’s not a lot of clear information about how to trigger them, and what rules and guidelines you should be considering in your implementation.
Part of the problem is that it has been a moving target for the past few years. At first, pretty much any site could achieve review stars in their snippet by simply adding the aggregateRating schema markup to their pages. Then, with the emergence of JSON LD, doing this got even easier — and implementation (and abuse) increased accordingly.