Hard (or impossible) compass calibration on 2.8

Hi everybody!
(From Italy)
I have a problem with the calibration of the compass. At the firs installation, everything was gone ok, but after the reset (for other reasons that I’m not explain in this topic), the compass wont to calibrate!
Is it helpful:
1- to fully reset the apm?
2- to try the calibration with the GPS locked?

thank you for your help!