Here's a quick test to see if your server is joinable. Use GIP or a program like JazzForce and check to see who's in your server. With GIP, you do this by clicking on a server name. If you can't see your own player's name within a short amount of time, then your server is most likely unjoinable.

If you can't find the name of your server, then your server either can't stay listed or isn't getting listed at all. I'm not sure how much of a problem servers getting unlisted is nowadays, but I suggest you check to see if your server is still listed every once in a while for the next time you host just to make sure that isn't the issue.