Seems the box to do so at the bottom of a thread is gone, at least for me. I'm going to notify GW.
Thought I was just missing seeing it at first, or that it might just be in one or two forums, but I've checked several now.
Unless the problem is unique to you and you are having a problem, you only have the box appear to post a follow up if you are logged on.
If you are not logged on to GW, then there will be no box to post the follow-up in. Instead you will see the words "log in" in red in place of the box.
Once you log in the box for follow-up will appear.