I have noticed that the nokia city lens is not working on my Lumia 920 when ever the phone is inserted into a case. I remove the phone from the case and try again the app and whola, the app is working, then I inserted it again on the case, then when I open the city lens it is asking for calibration, i tried to move the phone as instructed on the screen but no success at all. then again I remove the phone from the case and then It is again working... So im convinced that the compass of my phone is somehow affected by my cover case of the phone.