I think Todd picked a good reference there.
The solution may depend on how your report is formatted.
If you have defined a Report Header template (and it sounds like you probably have) try redefining the template to an append just while you see if the missed line come back when you do that.
If they do, can you shorten the number of SAMPLE data rows for the Page Header template by one? (In fact do you need a page header template at all - you don't have to have one for a model to work. The exception to that observation would be where you have a report with multiline records where the rows can appear above and below a page break. Then you are best to define a page header to make the header rows 'invisible' - which would give you the sort of result you are seeing now but only because you needed it!)
If you are unlucky enough to have a report where the number of Page Header section rows can vary a different approach may be necessary.