Which FileMaker version, and which OS ?
This post offers some suggestions, for a similar problem:
> the output pdf of file maker. the horizontal linesare not showing
Version 13.0v4, windows 8.1
In what viewer?
What if you zoom in?????
It's not unknown for hairlines to get rendered as LESS than one pixel wide so therefore won't always show
Adobe reader XI version 184.108.40.206,
why the lines not showing on the middle of the document.