1. Is it still loading?
The server is not in the list until it answers: about 15 seconds after a restart in our runs, about a minute and a half after the very first start (it downloads 2.8 GB first). On Flux the Overview tab says running when it is ready.
2. Search for the exact name
Search the list for the exact Server Name. A server that was restarted is a new session for a moment: refresh the list.
3. Who can join
A server limited to one platform (PC, PlayStation or Xbox) is not joinable from the others. Set Who Can Join to Crossplay in the Server Settings tab to let everyone in.
4. The ports
Both UDP ports (game and query) must be reachable, with the same numbers outside and inside. On Flux they are published for you and the server binds exactly those.
5. Same build
Players on a newer build than the server cannot join it. A server on Flux checks Steam every 30 minutes and restarts onto a new build when nobody is playing, or within the hour.
6. Full or locked
A server at its Max Players takes nobody else, and a password must be typed exactly. Both are in the Server Settings tab.
Frequently asked questions
Why is my Abiotic Factor server not in the server list?
Usually it is still starting (about 15 seconds per restart), it is limited to another platform than the player's, or the query port is not reachable.