There could be several reasons why your post/topic/reply isn't showing up >

1) to prevent spam, there are certain limits on the quantity and type of posts you can make - eg: unable to post any hyperlinks until user post threshold is reached. Such posts are held for moderating, and will appear when approved (this is not 24/7).

2) to maintain a speedy site, the caching is quite aggressive, so you may not see your post without refreshing the screen, or your post edit may not be immediately visible to you (but it will show in a private tab, and be visible for others).