I apologise if I am telling you hings you already know. Its been a while since I used the PM3 on the LF side of my NExTs, but I believe thy have to be perpendicular to the antenna to get a read. It often takes a few attempts as well. Are you using the LF antenna and not the HF one on the PM3? The LF is the visiible copper coil.
Otherwise, I will leave ti to the experts to reply, as I am sure they will be along in the next hour or two.