First please try turning your device off and on.
Next, please go to the Friends tab and click on the settings icon and check if the notification is turned on. The bar next to the 'Notification on' should be green.
If the 'Notification on' is turned on and you still do not get notifications, please press the Test push message button and check if you see the popup message, "Please wait a moment. Your request was sent to the server." followed by the popup message "Push notification seems to be working fine.".
If you do not see the popup message "Push notification seems to be working fine.", please go to the Friends tab and click on the Settings icon. Then click on Help center and submit a request.