I got the following feedback from players:
I have long game levels which can be played in more sessions. The content of the game can not be saved. Now this is what happens: The players click the pause button and turn off the device to continue playing when they return BUT: Sometimes the game still is open and can be continued but sometimes it is not and can't.
I now wonder what this can cause this and how can I have an influence on keeping the game opened in the background all the time, so players can continue a started (and paused) game whenever they want?