Why doesn't the button appear for a sold out product?
Back In Stock will not show the 'Email when available' button for a product that is out of stock, but can still be ordered. These are sometimes called pre-order products.
If you would like it appear for these products you can change the behaviour. Follow the instructions in the article enabling the button for preorder products to have the button appear.
If the Back in Stock button is not showing for a product and you cannot determine why - just get in touch with us on [email protected]