for those, like me, who upon opening the game, would never receive messages, or see who invaded/offered help, I have a useful tip that helped me.
I would enter the game in an offline state, by turning off WiFi and cell data. Then pressing home, turned both back on. Fast app switching back (hold the back button) I would force the game to connect to the server, usually just by tapping on the market. After a few seconds, all messages and invasions would pop in my town.