Sometimes achievements you earn are not displayed in Google Play achievement list. Soon we will change how achievements are synchronized, so this is mitigated. In the meantime, you can try this workaround to submit your achievements to Play Games manually.
How to submit an achievement manually
Make sure you're signed in to Google Play Games
Check the achievements icon in the main menu. If you can access the achievements list from there, you're properly logged in to Google Play Games.
Tap Start and Select a game slot. This should open the map.
Open the achievements list on the map (not in the main menu!).
Tap the icon of the achievement you've already earned but is missing in the service list.
The game will make the same sound it makes when an achievement is earned!