iReports - Background on sub-report does not display

iReports - Background on sub-report does not display

I'm using iReports 4.5.1 (unfortunately using a later version is not an
option) and have a report that has multiple sub-reports associated. In the
background band of the main report, I have placed text to be used as a
watermark - this displays fine on each report page. In one sub-report I
want to display an additional watermark, so have added an additional text
field element in the background band of this report.
When I run the report as a whole, the watermark text for the sub-report
does not display, yet when I run the sub-report separately, its watermark
text displays fine. Any ideas why this might be happening?
Many thanks!