Read more links don't work on any template

If I add this code <hr id="system-readmore" /> to an article, it doesn't render it (does not show a read more link anywhere). If I view source it isn't even visible there.
It does work on the home page though.
What can be the reason for this ? How do I get it to work ?

What Joomla version do you use?
And what is URL of your site?

After some further investigation, I found that I was being dumb, wanted to put a read more link on a plain full article, not a blog category etc. Sorry, my bad

Read more link not appearing for mobile version  , but it fetches all the content whereas its good on desktop.

Joomla version 1.5.20