The stars come from schema, not the page
Those star ratings in search results are 'rich results', and Google only shows them when it can read structured data (review schema) on your page that spells out the rating and how many reviews it's based on. If that schema isn't there or isn't valid, no stars, however many reviews you actually have.
So the reviews being visible to a human on the page isn't the same as being readable by Google. The machine needs the data in its own format.
The review app that outputs nothing readable
Most Shopify reviews come from an app. Some output proper schema, some render the reviews in a way Google struggles to read, and some need the setting switched on. It's common to have a page full of reviews and zero review schema in the code.
Check what your app actually outputs. If it supports rich-results schema, make sure it's enabled. If it doesn't, that's why you're invisible, and it may be worth switching to one that does.
Google's rules on which reviews count
Google tightened up on review stars: it wants reviews of a specific product or the business as reviewed by customers, not a business quoting its own overall rating on its homepage. Self-serving 'we're rated 5 stars' markup on the wrong page type gets ignored or can cause a warning.
Keep review schema on the product pages where the reviews genuinely belong, tied to the specific product, and you stay on the right side of the rules.
How to check it in five minutes
Run one of your review-heavy product pages through Google's Rich Results Test. It'll tell you whether it can see valid review or product schema, and flag any errors. That turns 'I don't know why the stars are gone' into a specific, fixable list.
If the test sees no review data, that's your answer: the schema isn't reaching Google, and the fix is the app or the markup, not more reviews.
More on ranking and on-page SEO: Why your Shopify product pages don't rank, and how to fix them · Why your Shopify collection pages don't rank, and how to fix them · How to write Shopify meta titles and descriptions that get clicks · Why your Shopify blog gets no search traffic · Are out-of-stock products hurting your Shopify SEO? · Why your Shopify store doesn't show up in local search · The Shopify SEO checklist that actually moves revenue.