I've researched this and I only found issues relating to the whole comment "area" (below posts) not appearing, but the issue I'm experiencing is different. (I refer to the blog as "my site" when I am actually referring to a friend's site.)

The comment area appears under each post, and when I submit a new comment, the comment does not appear in the admin panel for approval (comments need to be approved). The only slightly similar issue I've found that sounded similar to what I'm experiencing, was solved by enabling "Discussions" for specific posts. I can't seem to find the link now, but the person having the issue had just restored posts/comments from a backup, and needed to re-enable "Discussions" for each post one by one. I've checked the posts on my blog, and they all have Discussions enabled, so I don't think that is the issue.

One important note to mention, is that this seems to have started happening after I had my site "restored" a few months back. I'm assuming this issue is related to the comments not being saved/registered, but I'm not sure how I should debug this. Comments have worked fine in the past, and those that have been previously saved still appear on the site (and in the comments feed).

What is the appropriate way of enabling debugging for the Comment/Discussion functions, so as to understand where the issue lies? I have a technical understanding of WP/PHP/server administration, so feel free to ask anything.