EvelinaAdamovi is that a Wordpress post that existed prior to enabling the extension ? If so, this is the currently expected behavior. For now, the Flarum discussions are only created when the Wordpress post is created, or when the comments status is modified.
I have said before (but I can't find where) that I would consider adding an "import" feature if someone wants to install the extension on an existing Wordpress that already has many discussions. I have not implemented the feature yet because the extra development and maintenance work isn't worth it if no one needs the feature.
Another option would be to allow creating missing discussions on-demand, but this comes with its drawbacks. Without proper rate limiting users could abuse this feature to DOS the website depending on which extensions are enabled on the forum, in particular if there's a notification for new discussions.
For Wordpress posts that have been created since the extension is installed, the discussion should be created automatically at the same time as the Wordpress post. If that's not the case, there should be an error shown or logged somewhere when publishing the Wordpress post.