MS designed Access so the main report manages the page headers. I guess that
makes sense, as a subreport doesn't necessarily start at the head of a page.
If you do want to simulate a page header in a subreport, use a group header
and set its Repeat Section property to Yes. You will need a field that
doesn't change value so the group header continues throughout the
subreport's pages. You can achieve that by basing the subreport on a query,
and typing something like this into the Field row in query design:
AlwaysFalse: False