I have the Stratus3 and a 740. I don't know how Stratus reacts to iFly on an iOS device, but with the 740 you have to use the Appareo Horizon Pro app and set the "Open ADSB" switch "ON". IF the switch isn't ON, the iFly shows a connection, but will not display any data.
If you have ForeFlight, ony one program (ForeFlight OR iFly) can use the Stratus3 at a time.