My wife and I updated Mii Plaza on Friday and the new shop opened up. She bought the games about 2 hours before I did, played enough to get 2 of the hats, then played some more for the other 2.
I played enough of the games to buy all the new hats they had for the day.
The next day, my wife's shop updated with new hats. Mine didn't.
Yesterday, around 3, the shop updated for her again, mine updated for the first time and I was able to get the hats that she got on Saturday.
Today, mine still hasn't updated.
I was thinking it might be because I keep changing the date to get more coins since I intentionally didn't change the date on Sunday and it finally updated.

Anyone else having this issue with the shop not updating?

 

I tried that, no dice.

How often does the shop update for you?

 

The shop updates once per day, if I'm not mistaken. And changing the date might very well be the reason it didn't update for you, it would not be the first time such a feature is blocked from "cheating" in this way (the Gen V Pokémon games also have a similar protection for the Join Avenue).