Numbers of reason could have caused this such as:

1. It was caught up in our spam filter.
2. A moderator deleted your post.

But what can you do? Here are a few tips :

1. Message the channel's moderator and ask them the reason. They're usually friendly people and happy to help you.
2. Make sure you are checking the right channel.
3. In case you're experiencing the issue in your home feed, make sure to take a look at your feed settings.