It could be that your server is pinging, meaning the NAT settings within your local area network haven't properly been configured which prevents anyone from joining. If you did properly forward your ports, I believe the latest version of JJ2+ has various fixes in it that solves these problems.

Similarly, you might not be running the latest version of JJ2, which would make it impossible for anyone playing 1.23 to join your server. A simple enough fix is to run the 1.23 patcher.

Another likely reason is that there's no one else playing who wants to join your server because they're already playing on another or don't particularly care for the level or game mode you're hosting.