Why didn't I receive any Instant Payment Notification (IPN) posts?

If you’re having trouble receiving Instant Payment Notifications, review the following:

1. Your receiving account's email address is unconfirmed
   Check that the email address on the account the payment is being sent to is confirmed. For profile-based IPN, the email address on the receiving account must be confirmed, or else the IPN won't be sent to your script. 
2. IPN is disabled in your profile
   If you use profile-based IPN, check that it's not disabled in your account profile. If it is disabled, you may have received an email from PayPal indicating the server that hosts your IPN Notification URL is unresponsive or rejecting notifications. Once you have confirmed with your network administrator or developer that the issue has been resolved, review the instructions on how to enable IPN.
3. Your firewall is blocking IPN
   Instant Payment Notification is an HTTP POST initiated by PayPal. If you have a firewall on your server, check with your network administrator or developer that it's not blocking the post from PayPal. Find a list of PayPal’s live IP addresses.
4. Your IPN Listener URL has changed
   When using or switching from a third-party partner, shopping cart, CRM, plugin or platform, ensure that the IPN Listener URL has not changed. If it has, update the IPN Notification URL in your profile and click “Save” to begin service at the new URL.
5. Recurring Payments or Subscriptions
   Recurring and Subscription payments will only send to the original IPN Notification URL throughout the lifetime of the payment cycle. The URL is determined at the time of the initial profile creation. If the Notification URL requires updating for these profiles, please file a ticket with our Technical Support team and indicate the profile ID’s where the Notification URL requires updating.
6. Passing the ‘notify_url’ variable in your API request
   The notify_url variable passed in the API payment request will override your profile IPN Notification URL and may be different. Ensure that this variable is not passed in the API request with your developer in the event you want your profile IPN Notification URL to be notified.

See also: 
Instant Payment Notification Developer Guide 
Instant Payment Notification Simulator