The submission set to be delivered after payment does not arrive. Why?
Sometimes, in PayPal order forms, when the option Deliver only after payment is checked in Settings → Payments → Notifications, the submission does not arrive in your inbox at all. In order to fix this, please disable the option Auto return to website after payment in your PayPal settings.
To do this, log in to your PayPal account, click on Profile, then go to My Selling Tools.
Under the Selling online subsection, go to Website Preferences and click on Update. Under Website Payment Preferences set the auto-return on Off.