You will need to turn on IPN in your PayPal account and set the location of your IPN listener.
If PayPal, go to Account Settings > Website payments
Then in the line with the heading “Instant payment notifications” select “Update”.
When you click update, you will have a screen that indicates your IPN settings. Make sure IPN is turned on! If there is no notification URL, or it does not match the location indicated in the extension’s subscriptions tab, click edit settings and add this location in the Notification URL box. The image below shows these two screens. Again, make sure you’ve entered the correct IPN listener location for notifications and that you have enabled IPN messages.