I started on an iphone and they rolled in every 24 hours. On the ipad I've seen them only once and never again in days. A friend is having the same problem on an android tablet. Different programming? Any suggestions?
From the official Plants vs. Zombies 2 FAQ:
You must have an active Internet connection in order to play a round of Piñata Party. If you have received a notification that a Piñata Party is available to play, please wait until you are online to launch Piñata Party.
Perhaps you are not connected to the internet all the time?
If there is an iPad version of the app, fitted for the screen, then most likely it is different if you are running into this problem. If it is not an iPad version (how you can tell is if it has the zoom button in the corner) then the coding is the exact same. Sorry, I don't have this app but I try to contribute to the community and answer as much as I can.