Missing lines when I use Merge to Adobe PDF

Hello.

This is my first time posting and I hope I'm posting in the correct place.

I have a Word mail merge document which I convert to a PDF and I then send via Outlook using the Merge to Adobe PDF function in Word.

When I send the document a line in the heading disappears (this is in the headers section of the Word document) but the words remain.  It doesn't do this if I convert the document to a single pdf.

Anyone any ideas of why it does this as it didn't do it before. 

Thank you.