One issue that I have noticed with SSRS is that headers and footers do not contain query data that is derived from a hidden text field. 

The work around is to assign the data to a global parameter that is hidden.  Then reference the global parameter in the footer or header.  The data will now be available in Excel headers and footers. 

If you are not seeing a header or footer this is because by default Excel hides the headers and footers.  In Excel 2007, click the Insert menu then the Headers & Footers button.  In Excel 2003, click the View menu, then Headers & Footers.