1: On paypal can you see what HTTP response code you got? (click on some of the IPN History entries and look at the details).
If you get HTTP response 200 (ok), then it means paypal is successfully sending to the website and nothing is wrong with paypal.com configurations.
2: Also check znote_paypal table in your database, are there any logged errors there, or empty table?
3: When people donate, do you have to manually accept them on paypal? (If so it may happen that the ipn does not work intentionally, since the script is coded for instant payment only).