Having a review app installed and displaying stars visually on the page is not the same as having valid AggregateRating structured data in the page's schema markup. Google reads the markup, not the visual widget — if the review app isn't outputting correct schema, nothing shows up in search no matter how many reviews exist.
Run the live product page through Google's Rich Results Test. If AggregateRating isn't detected, that's the exact gap — check whether the review app has a schema markup setting that might be disabled, or whether the theme is overriding/stripping it.
Google requires a minimum review count before displaying stars in results (and reserves the right not to show them regardless) — a product with one or two reviews often won't display stars even with perfect markup. This is a reason to actively prompt for reviews on bestsellers specifically, not just hope they accumulate.