You can also train yourself to ignore the big orange box above, and use the "Reply" that is on individual people. You'll see one to my left, under my name, and under Dragonchilde's name, too.
It doesn't matter *which* reply button you use on the page. The thread doesn't record that you "replied to Unident" or "replied to Dragonchilde", your post just shows up next regardless. So go ahead and click the "Reply" that is besdie the name of the last person to post.
Checking for the other person's post below your empty box to write in will only work if you've hit Reply on somebody, rather than Reply on the top orange box. If you hit that Reply it's functionally the same - your post just goes next - but you don't see anybody's message below yours while constructing yours. So using the person's reply buttons is always best, especially as you can check before posting that their message shows beneath yours so it's definitely a Reply.
And as Dragonchilde said, if you have a Subject to fill in, you didn't hit Reply. You must have hit Create New Topic.
Deb, in New Zealand