I have 3 freeform datawindows, named A, B and C. C is a nested report of B, B is a nested report of A and A is a main report.
Report A is very simple, and has only one nested report B in it (in real life there are more, but one is enough to show my point).
Report B has a header, and in details band has many copies of C so that report B does not fit a single page (in real life there are many different reports, this is just for simplicity).
Now if I preview report B, everything is OK, I see the header on every page, and nicely arranged nested reports, but if I preview report A, on the second and later pages there is a gap the size of B header between first and second nested reports of that page: