If you look in your descr_strat file, you'll see this stuff.

playable
romans_julii
romans_brutii
romans_scipii
end
unlockable
egypt
seleucid
carthage
parthia
gauls
germans
britons
greek_cities
end
nonplayable
romans_senate
macedon
pontus
armenia
dacia
numidia
scythia
spain
thrace
slave
end

The ones under unlockable are the only factions that you can unlock in the game itself. The ones in the nonplayable are not unlockable and you must manually move those factions to the playable to make them work.