Compass which refuse to indicate the angle of my rover

Hello everyone,
I’m having a problem with my compass which seems to refuse to indicate the angle of my rover correctly. I’m using a MatekSys CAN-L4-3100 (based on RM3100). I’ve flashed it with the version of AP_Priph that allows it to communicate in MSP (my FC has no CAN). I know that running the motor can disturb my rover as well as the batteries. I plan to move the compass further away, but for the moment I’d like to make it more efficient in this configuration. The disturbance to the motors is normally slight, isn’t it? I have a very/too strong phase shift for it to come from there. A little additional information: my compass has been calibrated using “Large Vehicule Configuration”.