Some levels of the game can be tough to tackle and after all your hard work, the satisfaction of the win can only be wiped out if you come back to play the game and have to tackle the level again, so it's something you can easily avoid.
It is probably best to start by knowing that the game recognizes your completion of a level at the very end of the achievement. You should see a final 'confirmation message' to recognize you've completed the level and you must allow the confirmation message to show, for the software to understand it's been achieved. It's like putting the full-stop at the end of the sentence.
This 'bookmark' ensures that when you go back, the game knows you've completed the game, but also that our server which stores your player progression can see it too. Then you can play on another device, a new handset or you can uninstall and reinstall the game, and you'll never have to play the same level twice.
If, for any reason, you switch applications or close the game before the completion confirmation completes - it's possible this doesn't allow the marker to be added. The game was completed, but the 'full-stop' wasn't added at the end. It's important you allow the 'completion message' to show, so as you don't have to play a level again.
When in doubt, drop back to the map screen or open the next level and you can be sure your hard work has been recognised and logged. No more painful deja vu!
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