There's lots of poorly calibrated compasses out there. The first thing I would do if dependent on an ADF is get the compass properly swung.
On my IFR ride, I did an NDB approach and when it came time to lift the hood, the field was a mile to my right -- and the examiner passed me. Well, I had followed the needles correctly, even if they pointed in the wrong direction
I once used a back bearing for a considerable distance over unsettled country and found myself considerably offset from where I thought I was going.
Newly installed equipment can do funny things to the compass. My instructor and I once entertained ourselves by seeing which radio moved the compass needle the most when switched on and off. The ILS receiver moved the compass 20 degrees which made tracking to the LOM interesting.