I use Sublime Text 2 and want a Ruler to be shown in every file with specific line-height. But I have to show it manually in every file.
I just had this problem and noticed that the my ruler was only showing up when I was slightly scrolled to the right horizontally. Turns out the reason it was disappearing was because I was slightly zoomed in. Press Cmd+0
to make sure you are zoomed to the default 100% size before trying other things in case this is also your issue.
(I am using Sublime Text 3)