Also, before you submit the changes to the initial post, make sure you put the contents of the box where you type into the clipboard. (Ctrl+A then Ctrl+C as soon as you finish typing, before using the mouse again, is a quick & easy way.

)
I'm suggesting this because sometimes when you edit the initial post of a thread, it changes the thread's title to "0", and blanks out the initial post.

(If it's already been blanked out, the submitted edit is successful.

)