Demo playback fails if:
The map was not found
A required mod was not found
The Network Protocol version of Quake that saved the demo was different from your current version
NO demos show up on the demo menu and if I use console to playback, it loads up the map and just as it should start playing, it kicks back to the main menu. Not sure why.
You have a different version of the game than when it was recorded. For example, if it was recorded with Q3A 1.17 and then you later upgraded to 1.32, or vice versa.