Last month a client's WooCommerce store lost every product thumbnail overnight. Nothing in the admin looked wrong, and the Media Library was full of images. The cause turned out to be a delay-JavaScript option in an optimization plugin hiding the product gallery until someone interacted with the page.
That is the pattern with missing product images: the file is almost never the problem.
My quick test is to open the image URL directly in the browser. If it loads, stop looking at uploads and start looking at what stands between your server and the visitor: cache layers, lazy loading, WebP rules, a CDN, or a script that reveals the gallery. If it returns 404, the file or its URL is wrong, often a leftover from a migration or an http to https switch. If it returns 403, check permissions, hotlink protection and security plugin rules.
Two smaller things catch people out. First, a product needs a featured image and not just gallery images, or many themes show a placeholder on the shop page. Second, after a theme change the thumbnail sizes the new theme expects may never have been generated, so regenerate them before blaming anything else.
Conflict testing comes last, on staging: default theme first, then plugins one at a time.
I wrote the complete ten-step order, with a causes table and FAQs, here: https://amanurrahman.com/blog-post/woocommerce-product-images-not-showing-fix