It's strange that it works "sometimes". As an alternative / fallback you can always give them the IP and they can join manually without using the list.
What do you mean with this?
The dedicated server, dedicates everything about your server in an one program, and it trys the best to comfort you and to host the server in the same time.
Also an additional thing: Make sure, you corectly and propely configured your router (if you have one) and opening all ports (Port Forward) to let your friends or other players to join in your server.
In the future, if you still have some problems regarding your server that your friends or players can't see or can't join your server check the CS2D FAQ - Nobody joins/sees my server.