Amy, I have not run into this issue myself, however I have had some crazy things happen when using rdl files. My suggestion was based on what I could find on short notice, not an actual,tested,solution. I have, for all practicalities, no experience using rdlc files. I don't know what the differences would be.
So you have made all these changes, and still the whitespace. Is it the same size, smaller, larger? What happens when you leave something in there that is very small? What happens if you turn off the header completely? What about using the code that I provided up there but change it so something would show up on every page other than the first page, in essence creating your own 'report header' logic?