I think the sensitivity of the sensor stage is insufficient. In the calibration did you obtain self oscillation with the quartz oscillator disconnected? I attach the helps and add, perhaps it wasn't understood in the helps) that at the beginning you have to obtain the self oscillation and therefore you have to decrease the gain (via C13 and C14) until it disappears.
The maximum sensitivity in our hemisphere is in the north/south direction and the minimum is south/north.
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