InfernoDragon posted... No, they always eventually show up. At least one of the encounters takes as long as two in-game weeks (from the one before it) to appear. You might just need to wait longer in-game.
If that's not it, go back and check the other ones. Is there any chance that after you got the achievement, you didn't save?
Thanks for clearing that up. I'll just have to poke around the city a good while longer then. I wish I hadn't finished everything else because now I just have to drive around doing nothing.