Well if you can recognize patterns, you will realize that there is a reason why it sometimes does not. That is why the preview button is indispensable.
The pattern is this: If you have an error in your post (forget a / or a [ ]) then obviously it will not work.
If you preview and find such an error, then correct the error and attempt to post - it will not work. You need to copy your entire post to the clipboard, then you need to exit the post window and re-enter it from the thread (clicking on the name of the thread at the top and then hitting reply is all you need to do).
Then, if you have indeed corrected the errors in your post, it will work every single time.