Despite the prevalence of use of phpBB3, some people still aren't used to one of the basic anti-spam features that acts as a first line of defense against spam robots.
Basically, if your post isn't showing up right away, it's because your post has been entered into a moderation queue. So what does that mean? It may take a while for your post to show up. Just because you don't see it doesn't mean it's not there - if you get the message that your post was successful you can rest assured that it is. Your post will be approved as quickly as possible by a moderator.
Sorry if this is an inconvenience for you, but it is there. Hopefully this will help clear up any confusion. If you need to edit a previous post but cannot see it, you are free post your message a second time, in edited form, and if you wish to avoid confusion, inform the moderators inside your post that the original is obsolete. They will find it and take care of it accordingly.