Mk179

I seem to have a problem with my mk179 proximity card reader. The cardreader only seems to be able to read the card up to a distance of 2 cm. While the instructions say till 5 cm.
Could this be a problem with the 12 volt dc power supply, that it isn’t delivering enough power and thereby affects the effective distance? Or could it be the distance between the reader and the motherboard? Any ideas and explanations would be usefull…