I've had a quick look at the code, it does seem to be using the autoresponse postback from worldpay. So look in the Logfile to see if you have "AUTO SendRepsonse:" as any of the entries. Anythng in there might help you understand the issue.
https://nbstore.codeplex.com/wikipage?title=Audit%20Logs
https://nbstore.codeplex.com/wikipage?title=Audit%20Logs