If you swap phones and restore your phone from a backup or some other issue and your push notification messages stop working, the first thing to do is log out of the app and log back in again which will make sure your phone gets registered with our server. The next step is making sure that you have the notifications enabled in the alerts page of the app. Here's a quick video showing these steps.
If its still failing to send notifications after that, then uninstalling the app, restarting the phone and THEN reinstalling the app should do the trick. If not, please contact us.