Simple Catch says Comments are on, but "Comments Off" on front end

Let me start by admitting I’m nearly clueless with WordPress. This may be a simple “tick the box that allows all Comments” kind of answer, and if so, thank you/sorry! ; )

So, inside the Admin, it looks like Comments should be allowed on all the posts. At the bottom of the post, under DISCUSSION, the “allow Comments” box is ticked. (and so is the Pingback/Trackback option – should it be?)

But in the front end, the post says “Comments Off”

Even more strangely, some other posts in the blog have comments that are working.

Is there something obvious I’ve done incorrectly? I don’t recall anything different in the posting procedure.