Hi,
I must admit I also don’t know why this could be ahppening. Do any errors show up in the log? If you set the logging level to trace, you will get some additional information which might be usefull to troubleshoot the issue.
Perhaps it could help to look at the temporary Postscript file(s) which are created in %temp%\PDFCreator\spool prior to the actual conversion.
Best regards
Robin