By default a "Reply" link should appear on every post next to Edit, which means that somewhere in your style something was changed.
see:
without a link to the forum in question, there really is no way to tell what's going on..
EDIT: Check your Quick Reply settings, if you turn Quick Reply off, the reply link will disappear (at least it did for me on my dev vBulletin)
This = No Reply link at bottom of post:
This = Reply link at bottom of post:
the third option should be the same as the second image.