I have an A2 flight controller installed on an S900 with a Zenmuse Z15-N. It will not complete a compass calibration. The LED changes from blue to green to flashing red. I have tried everything I can think of, including replacing the GPS Pro Plus module. Needless to say, the firmware is current, and during the many, many attempts sufficient satellites were in view and no ferrous metals nearby. After several months and countless hours of troubleshooting, I'm out of ideas. Can anyone help?

I have the exact same problem and ive written into DJI about my pro lem and they have been zero help. I was a real firm believer in the A2 system. I put them in a dozen multirotors. However i purchased 5 A2 complere flight controllers and 3 of the 5 are responding the exact same way!!!!!!! So i have (2) Tarot Iron Man 1200mm octocopters and a DJI S900 that i cant sell cause no matter what i do, i cant pass the verrical compass calibration.  Any assistance would be of tremendous help.