If you take a break from playing the game in the middle of a level to do other things (checking your email, feed the dog, and so on), your device may secretly force the game to stop running in the background to conserve memory and battery life.
When this happens, upon returning to the game the level you were playing will restart from the beginning, and the life you used will be gone.
The duration of time before the device does this depends on the device, and we have no control over this. For this reason, we recommend you finish up anything important you were doing in the game before switching to another app or putting your device to sleep.