My product has reviews, so why are no rating stars sent to Google?

Check that Rating stars in search results (AggregateRating) is on in Plugins → AI Search Optimization, then click Save Changes.

The rating is only added when the product page has at least one review with an average above zero. A product with no reviews gets no rating at all, because Google penalises a rating that does not match what shoppers can see.

Even with the markup in place, Google decides when to show stars in its results. It can take time after Google next crawls the page.