Recently, I received 100 rep because one of my accounts hit 200+ reputation. All the accounts received this addition, except Stack Overflow. Why not?
You did receive the association bonus. Take a look at https://stackoverflow.com/reputation, which provides a detailed breakdown of your reputation score. See at the top where it says
--bonuses (100)? That's the association bonus that's been granted to you.
I would suggest emailing firstname.lastname@example.org They're quite responsive and can probably address this quite quickly.