Am facing the same problem.
We are getting back the all variables through IPN when test the purchase using sandbox account, but with live account we are getting null result, so we cant get the purchased items details for our database, pay pal is returning successful transaction email.
i have checked the code, we have replaced all the sandbox links with original one, Original account have ipn and returning URL settings same as given in sandbox account.
we are using codeIgnitor for development.
Any Help will be much appreciated.