1. It is still starting
A new server downloads about 12 GB and builds the world before it registers, which can take up to half an hour. The server log says "Server has completed startup and is now advertising for join" at the moment it becomes listable. On Flux, the dashboard's Console tab shows that log.
2. The filters
In Join Game: the Unofficial filter, Show Player Servers ticked, and the search set to the server's exact name.
3. Crossplay, for console players
PlayStation 5 and Xbox players only see servers started with -ServerPlatform=ALL. On Flux that is "Who can join" in Deployment Settings.
4. The port
The server registers the port it bound. If an outside port is mapped onto a different inside one, the listing points at a port nobody can reach. On Flux the two are always the same number.
5. A paid map
Astraeos and Lost Colony are paid DLC. Players who do not own the map cannot join a server running it.
Frequently asked questions
How long until a new ARK: Survival Ascended server appears?
On its first start, up to about half an hour while it downloads and builds the world. The log line "advertising for join" marks the moment it can be found.
Why can PC players see my ARK server but console players cannot?
Crossplay is off. Start the server with -ServerPlatform=ALL; on Flux, set "Who can join" to crossplay in Deployment Settings.