Hi. Sure - because by default K2 don't support FB comments. If you want to use FB comments on K2 items you have to read more about it on K2 community forum or/and use FB comment extension.
I can suggest to use our : Social GK5 module - and try to publish it on position like mainbody_bottom then the module should be displayed after article content.
Hi, if you use the form gk social, comments do not differentiate between the various items, or am I wrong? I then used a script that I placed on the page item.php of k2.