Just got my P3P and having one hell of a time calibrating the compass.  I think I've done it the way it is supposed to be done, but it won't calibrate...

are you doing it as it says in this video? (around 5 minutes in) sometimes if you keep on spinning a bit further than 360 it helps to get a solid calibration...

http://www.youtube.com/embed/HoTdK7izUAA

Best place is a big lawn no metal around or powerlines above. Or metal pipes in the ground even.

Try different spots, drive ways and parking lots can be bad.

Don't wear any metal, like big belt buckle, and leave the cel phone with the remote controller 20-30' away.

If it still does not work contact DJI, or your dealer.

Can you please describe your symptoms of calibration failure, i.e. the leds fail to flash green as you rotate the P3, etc. In the GO App, under MC Settings is a menu labeled Sensors. What are your compass sensor values as well as the MOD value?
In addition to the link provided earlier, here is a library of links about compass calibration.
http://www.phantompilots.com/thr ... plete-primer.32829/

http://www.p3groundschool.com/pilot/compass-calibration

I hold it over my head at arms length.  The lights will tell you when you have turned enough for both moves.  Keep turning until the lights signal you to change from horizontal mode to straight down mode, and turn again with the front pointed straight down toward you, turning until the lights signal the rotation is enough.  Put it down and go fly.