if i select epic map and 5+ factions they either don't spawn or never expand?

I'm not sure I've ever played on an epic-sized map (I'm certain I've never completely reclaimed one, anyway), but it is really enormous. I know factions tend to expand slowly but steadily, so maybe they're all just out toward the edges somewhere, and you haven't run into them yet? If you're playing on a higher difficulty, that's likely stunted your own speed of expansion a bit, what with having to spend so much time & tie up so many survivors killing/defending against zombies (not to mention managing your food flow).
I've played this game a lot, and while it's not exactly bug-free, I can't say I've ever had an issue with factions failing to either expand at all, or to appear in the city in the first place.