RM3100 won't calibrate

Hi all.
Running into a problem with an RM3100 on Arductoper 4.0.5.
I have a few of these now on various drones and they have cleaned up a lot of compass issues…so very grateful to have them.
However I have one drone I can’t get it to calibrate on.
IT was originally mounted at the far end of the drone near the gimbal. No luck.
I thought perhaps it was the gimbal so I unplugged it. No luck
I then replaced it with a second one I had and moved it to a different location. That worked.
I then went back to the first compass in the same new location…That worked.
I plugged the gimbal in and tried again and no go.
I don’t know enough about these to understand whats up. Magnetic interference…Electrical noise.

Any thoughts from the community would be appreciated.