Sometimes it may be necessary to debug why IPN appears not to work.
You can simulate IPN following this guide .
If you want to see more detailed activity you can specify a debug file by adding a definition to your wp-config.php file.
Edit your wp-config.php and add the following line and a log file will be created, you can choose any file name / path you like.
define( 'IPN_DEBUG_LOG_FILE', 'ipn.log' );