It’s most likely that your store has a custom add-to-cart flow that is preventing the pop-up from initiating. If it makes sense for your use of the app, you may want to try changing the setting to trigger the pop-up “When checking out”. This often converts better anyway.
If your use-case makes it necessary to trigger the pop-up upon adding to cart and it’s not working, send us an email here and we will review your website and provide instructions on getting it to work.