Okay so you have tuned the PM3 and then moved the LF xFD all around the proxmark LF antenna while running a command like lf search or lf hid watch to power the coil? If so, I would usually say the LF antenna is having issues (a possibility with anything but especially knockoffs like the currently available easy) but you got a read out of a separate card right? Given you can read a normal tag and but you can’t light the xLED I do not know what to think, it could be a bad coil that just happens to work enough to pick up the larger tag but not having an easy I may leave this to other forum members who have more hands on experience with your hardware.
Regardless have a look at this post for more info on positioning it may be helpful if you have not already seen it: