I've failed to reply to a thread twice this afternoon. I have given up and the world will not benefit from my wisdom;.
The first time was my fault. I typed a detailed and informative reply in the quick reply box then clicked the post thread button which opened up a new page with a blank box.
Second time I was more careful. I worded a slightly shorter reply and pressed the reply button. Took me to a new page with a blank box.
I'm guessing in hindsight I ought to have pressed one of the white buttons instead?
Usability is all about just making things work. If you read this I got lucky. Perhaps moving the buttons around and altering colours might make things more obvious.