I have a layout container, then a header, then the render tool which creates a spreadsheet. In the render tool I have ticked the Header box and selected my header, then ticked show on first page of report but its not coming through!
Are you outputting multiple pages (using a custom field in the Layout Tool)? What defines the first page? Try all your worksheets to see if it has placed the header on a different sheet. The pages are output in alphabetical order if I remember correctly. If you untick that box "Show Header on first page" then does it show up?
Try playing with the Paper Size and Margins in the Render Tool. Perhaps there is not enough space for the header.
Feel free to send over a masked/sanitized version of the workflow so we (the community) can take a look → much easier to solve.