This may sound like a stupid answer but trust me, I've done it myself many times.

Make sure that you are signed in.

I have the Round Table in my Favourites bar across the top and sometimes when I click on it I get taken to the RT already signed in and sometimes not. If not, the reply button isn't there and I have to go to the top of the page and sign in before the reply button appears.

That's using fully updated Firefox on a Mac.