For back in stock to show - these conditions must apply:
(0) Most importantly - the app needs to be enabled in your store!
Themes 1.0 guide - https://support.apphub.com/en/articles/6269583-themes-1-0-how-to-enable-pre-order-bear-in-your-theme
Themes 2.0 guide - https://support.apphub.com/en/articles/6269581-themes-2-0-how-to-enable-pre-order-bear-in-your-theme
(1) The product is included inside your back in stock targeting rules inside the app
(2) The product is out of stock in your products backend
(3) The product is marked as "track stock" AND is not marked as "continue selling when out of stock"
Still not showing?
You might need to add a custom selector to help the app detect the product form in your product page.