Please try these two potential solutions:

 

1. In your device's Settings, sign out out of your Google Play (Android device) or GameCenter (iOS device) account on your device, then log back in.

 

2. If the above does not work, Please clear your Google Play or Gamecenter App caches:

 

[Android Devices] Settings > Apps > Google Play Services > Storage & Cache > Clear Cache

And/Or

Phone Settings > Apps > Google Play Games > Storage & Cache > Clear Cache as well

 

Always use a reliable Wi-Fi connection rather than 3G or 4G, if you have the option. 

Even if you have good signal strength showing for your Wi-Fi connection, it's possible that your bandwidth may be limited.

If the network is congested - for example, if there are many other users sharing the network or bandwidth-intensive activities are happening (such as streaming video) you may experience lag.