Previous and Next links not functioning properly

<div class=”prevnext” align=”center”><?php posts_nav_link(‘sep’,'<-‘,’->’); ?> </div>

This is for this site:

http://iloveyoumorethanblank.com

each heart is a post displayed via the_content, there are A LOT of posts and the idea is to have prev/next take you to the next few hundred hearts. It is not working. It changes some hearts in the middle but the most recent and the oldest on the page stay the same. I am pretty handy with WP and have tried and tried to figure it out.

Any help is greatly appreciated.