I don't know why, but it happened to me, too. And I'm a newbie so I was really confused! Seems it's some sort of 'control' GW has so that people don't double post maybe? If you are going to post soon after you just posted in the same thread, you need to change the subject of that post (editable box above message box) and then it will let you post.

Hi!

If you post more than twice or so to your own post, you will need to change the title in order to post again. I have no idea of why this is, but just change your title and try it again.

What mom2sethc said. If you post more than one post to a thread in succession too quickly, the software rejects it unless you change the title. This happens whether it's your thread or not, and what "too quickly" means seems to vary considerably from day to day.