Hi.
The Android device must have a magnetometer. If magnetometer is not present in the device, the input motion values are not calculated. This is happening in the lastest versions of the engine. To know if your device has a magnetometer, install a compass app on it. If there is not a magnetometer, the app will fail. If you need to get input motion values in devices without a magnetometer, the GameActivity.java template must be modificated. Take a look to this issue: Unreal Engine Issues and Bug Tracker (UE-59537)
Regards.