Hi Gav la,
Please run the Retro-Notifications Report (Enhanced) - PDF report, which will show you the element is marked to be processed in retropay process and subsequently run the retropay process, if the process didnot create any retro entries,please check the retropay logs including slave processes logs, which will show you the reason for the elemnt not being processed or the reasons of the issue.
Hope this helps you.
Cheers mate
Sudhir