I have my iPad Air mounted in an Apple smart case. It has the standard magnet along the lip of the cover that shuts off the iPad when it's closed. But, a couple of times now, it hasn't shut off my iPad. I've closed the cover completely in the evening then opened it the next morning to find the screen still on and the battery worn down to 10 - 15%. The magnet on the lip of the smart case cover is strong. (It really sticks to my fridge). It closes completely and the cover sits flat, but it doesn't always shut off the screen. Is this normal? Has anyone else experienced this?