How to debug a recurring subscription that is not updating membership
civicrm_system_log contains records of all the IPNs that Civi gets.  See if it's present there.
PayPal IPNs log quite a bit of data in the CiviCRM log (in the ConfigAndLog directory), even on success.  Find the log entry and look for errors.
You can replay IPNs on CiviCRM 4.5+ using the "notificationlog" extension: Is it possible to do an IPN replay?
In some edge cases, it's worth checking your web server logs to correlate the call to extern/ipn.php with PayPal - I've seen a case where PayPal reported success on delivering the IPN - but it was to the wrong site.
You can resend IPNs from PayPal for a certain number of days after the original: https://hq.palantetech.coop/projects/commons/wiki/PayPal#Resending-an-IPN