Symptoms
PayPal Instant Payment Notification (IPN) is not alerting my store to new PayPal orders.
Cause
If the IPN notice fails 5 times, PayPal turns the feature off and stops attempting to notify stores of new purchases.
Solution
Log into your PayPal account and verify that the notification URL for PayPal is set properly (under Profile->Selling Preferences->Instant Payment Notification Preferences). This should be set to http://www.yourstore.com/paypalnotification.aspx by default. You should also verify that the page is publicly accessible so that the PayPal callback is not blocked by a routing/authentication issue.