On iOS, you should click on device settings, then on notifications, find Top Eleven and turn notifications on.
If you play on Android phone, try to transfer Top Eleven application from your SD-card to internal phone memory. Also, if you close the game by clicking on "Back" button and confirm, you will stop receiving notifications. In order to prevent this you should exit the game by clicking on "Home" button.

My notifications still aren't working as Top11 Android app is in my phone memory and I always exit through the Home button.