Not able to calibrate RM3100 compass

I am building a new Antenna Tracker at the moment and I would like to use at least one RM3100 compass as I found them very reliable in Copter enviroment.
When I connect the RM3100 to an Pixhawk 2.4.8 it gets recognised but if I click on the “Onboard mag calibration” button, nothing happens. I had the same problem with an compass (different type but I forgot which one) build inside a drotek GPS. So I had a workaround: I changed the COMPASS_LEARN value to 3 (InFlight-Learning) and it worked for the drotek compass but not for the RM3100.
Has anyone experienced similar problems? What could be the reason?