Unable to start onboard mag calibration

I’m unable to start an “Onboard Mag Calibration” while being outdoors. The percentange remains fixed at 0%. However, when I previously tried the procedure indoors, everything worked as expected, the green bar was progressing as I was rotating the aircraft. Someone could help me to understand this behaviour? It seems like whenever I have GPS lock, I’m unable to do the “Onboard Mag Calibration”. I’ve also tried the “Large Vehicle MagCal” and no errors, but I’d like to perform the other one.

Please, someone that can provide at least some clue. I don’t know what else to try. The official docs are not very detailed about the differences between the available compass calibration options, nor the technical details of the process itself. If any additional information is needed, just let me know.

Thanks in advance