The reason is twofold:
a. Your android phone is most likely not in airplane mode: It transfers data to the iPhone by first setting up a WiFi hotspot on the iPhone to which the Android phone connects. The problem is that the wifi hotspot does not provide internet access. So in the background, Android tries to shut down access to this hotspot, resulting in the Android phone being unable to access the iPhone. Putting the Android in airplane mode solves it on most phones. Atleast it did on our 1+1. Lots of Apple.com threads say the same things.
b. Lack of permissions: On many devices, it doesn’t gain the permissions upfront. You need to open the privacy control app (or the equivalent) to give the app full access.