So, at the risk of sounding like a dummy__why do some posts have a reply key while others do not?
The reply key is at the top of the page. When you click on it, a box will open for you to post a comment. It doesn't mean you're replying directly to any particular post. If you want to reply to an individual post, use the quote key which you will find attached to each post.
It confused the beheebies out of me too in the beginning. It's still pretty much trial and error for all of us.