Hmm.. I'm assuming this code is all in a separate comments.php file? I think you may be missing this bit of code at the end of your comments.php file after you last <?php endif; ?>:
<?php comment_form(); ?>
Tough to say for me to be sure without looking at your actual files.
I can't get my comments to post on my wordpress theme: http://lwebdesigns.net/blog/?p=26#respond It just brings back an error page. The code is here: http://codepen.io/chibiwawa/pen/lhase
I also can't get the excerpt function to work: http://codepen.io/chibiwawa/pen/nwjuD
Hmm.. I'm assuming this code is all in a separate comments.php file? I think you may be missing this bit of code at the end of your comments.php file after you last <?php endif; ?>:
Tough to say for me to be sure without looking at your actual files.