page breaks not breaking anywhere

So there i repeat the issue – because i still could not find any way to solve it, neither at different hosts nor with different, even the most advanced themes.
For instance, single.php has
“<?php wp_link_pages( array( ‘before’ => ‘<div class=”page-link”><span>’ . __( ‘Pages:’, ‘tempera’ ), ‘after’ => ‘</span></div>’ ) ); ?>”
in it.
But, “<!––nextpage––>” put into the post in plain text editing mode does not result in any page breaks, at none of the themes or hosting i tried.

Someone having a clue why not??

I also use WP Page Navi plugin, but i doubt this has any influence as i tested the next page thing also on sites where i do not use it, just totally baffled how come i can’t get this simple thing to work …