This isn’t the first time I’ve seen a problem like this, though I was under the impression that this problem had been recently fixed. I’ll forward this on to the web team.
In your case it is because you are a new user. All posts by new users go through a moderation queue and must be manually approved before becoming visible - this is how we filter out spammers, who are a persistent problem. Once you have a couple of approved posts, you’re assumed to be a real person and therefore no longer need to be moderated.