If you are using custom buttons for Add to Cart, then the popup won't appear. The Add to Cart popup appears with the default Add to Cart buttons which must have standard WooCommerce classes. The button class should be the 'single_add_to_cart_button' and you will also need to add the 'data-product_id' attribute to that button to show the Add to cart popup on the Add to Cart button.

It is also possible that you are using the Add to Cart button on a custom page & not one of the default pages. So in this case, the custom page needs to be added under the option "Custom pages to display the pop-up modal on" under the Add to cart Popup Editor section.

If this does not help then you can contact our support team to get this resolved as each custom button needs a different solution.