I have the same issue with my iPad mini. I have both an iPhone 6s, iPhone 5 and iPad mini 2. All are running up to date iOS, on the same wifi network and have the correct date and time. I regularly misplace all
l devices. If I'm looking for the phones, it's not a problem. I use Find My iPhone, locate them, play a sound and hunt them down in the house. The problem is the iPad. If it is sleeping, it will appear as "offline" in Find My iPhone. If I play a sound, nothing happens. I roam the house until I find my iPad on my own. As soon as I press the home button, the find my iPhone alert starts playing. The iPad can only be located while awake. The iPad does not have cellular data. Does the cellular network keep iPhones a tIndy bit more awake (connected to the system) when sleeping? It's almost like he phones have a secret "Wake for network access" setting and my iPad doesn't.