There are several potential reasons:
- Have you configured a Game Server Login Token (GSLT)? This is required for visibility on the Internet server list.
- Are players outside your network able to directly connect? If not, you may need to configure Port Forwarding to allow external traffic before the server will be visible and joinable.
- If players outside your network are able to directly connect then it may simply be a matter of time before it becomes visible. The Steam master server is a "black box" so the exact implementation is a mystery, but it may depend whether the server has been online for a certain amount of time and/or has active players. In particular when there are a large number of active servers with players on them it seems empty servers might not be returned unless specifically filtering for empty servers.
- If your server was previously visible but now has a red warning on the lobby screen a reason should be listed on the Unturned Server Standing page.