comment_form() doesn't work

I currently am using the old school code for adding comments in comments.php, but I want to clean things up and modernize the site by replacing all the code with this:

<?php comment_form(); ?>

I put it into the comments.php file. The form for entering the comment works fine, but no actual comments appear. All the comments are approved, yet all I ever see is the form for adding a new comment, I never see any actual comments.

Once I revert back to my old code the comments show up again. It’s my understanding that the single line of code above is all I need for a basic commenting system, and I’ve read the codex and tutorials on the web and it confirms that it’s the only required bit of code.

Can anyone help? Thanks!