I'm not sure why your macro might be omitting part of the page. As for the blank pages, that's because you're inserting the Section break that goes with each letter.
Might I suggest you discard the current process and instead drive the merge with the '
Send Mailmerge Output to Individual Files' macro here:
https://www.msofficeforums.com/mail-...ps-tricks.html. With that macro, your individual files will be generated when you do the merge and their formatting will be preserved. As an added bonus, the output can be saved as PDFs and/or docx files at the same time.