Hi Albert,
It may be problem with Firstdata address validation, if your customer enters the invalid address.
It will show success code, but transaction gets failed. You can disable address validation form your site files and FirstData account.
In Firstdata account, go to Administration >> Terminals >> select the terminal >> AVS Filters (you can disable the address validation fileds).
If you still face this issue, kindly send your site FTP and backend to support@modulebazaar.com
Regards,