if you know when the subreport is to fire, you might try changing the page header in your main report to match.
If your subreport is that long, it might profit you to use a stored proc and bring all your data together into a really large table, this way you can probably better program the report to appear correct.
I think that subreports were designed to be small, less than a page, which is why they only have a Report header section.
Just a guess.