Scaro,
It may not be a problem with your installation but on one ADF we installed the vendor-supplied attachment screws for the antenna were anodized & thus insulated. The install required conductive screws to carry the ground plane through to the underlying metal - changed the screws & fixed the ADF.
Some ADFs (Collins' ADF-462 for instance) have QE Corrections that can be strapped in by program pin but I doubt that's on you GA receiver.