Why isn't the Notify me button showing on my sold-out product? (Essent Preorder Back in Stock app)

The Notify me button shows only when the selected variant has 0 or less available stock and the product is covered by a Live Back in Stock campaign.

Check these

1. App embed is on for the theme you are viewing (Online Store → Themes → Customize → App embeds).
2. The campaign is Live and covers the product. In the Back in Stock campaign, open the Back in stock products tab and check it targets All products, the right collection, or this product.
3. Stock is really 0. Committed stock still counts as available. The button works per variant, so pick the sold-out size or colour on the page.
4. Add to cart still showing next to it? "Continue selling when out of stock" is on for that variant, so Shopify keeps selling it. Turn it off under Products → Inventory if you only want Notify me.
5. Your theme's own "Notify me" or sold-out block isn't showing instead. Turn it off in the theme editor.
6. Check the live store in an incognito window.

If a product is also in a preorder campaign, the Back in Stock campaign's setting If a product is in preorder and back in stock campaigns decides whether both buttons show or only Preorder.

Full guide: Enable Back in Stock notifications in your store.