Some levels can be tough to tackle and after all your hard work, the last thing you want is to have to do it all over again! But don't worry, this can easily be avoided.
Please note that the game only 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 register it.
This also ensures 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 displays, this can prevent your progress from being saved. You must allow the 'completion message' to show, to avoid this.
When in doubt, go back to the map screen or open the next level to make sure your hard work has been recognised and logged. No more painful deja vu!