I have an iPad Air 2 with a Smart Cover (not case, but should be equivalent for this purpose). I've noticed that it no longer works consistently in iOS 9. Sometimes - maybe 10% of the time - opening the cover does not cause the screen to illuminate. When it's in this mode, I can open and close the cover as many times as a like, and nothing will happen. A tap on the home button wakes it up, though, and then things work fine.
I suspect it's a software bug, but it could be the iPad hardware going bad...