I just spent a little time getting some readings with my Proxmark3 Easy and my xEM implants. I was able to determine what works (reasonably) reliably and what doesn’t.
In the diagram below, the blue circle is the antenna, the red lines are implant/chippositions that don’t work and the green lines are implant/chip positions that do work. Essentially, the antenna needs to cross about the middle of the implant/chip in order to get a successful read.
If the implant/chip is in the middle of the antenna circle, it doesn’t read.
If the end of the implant/chip is at the antenna, it doesn’t read.
If the implant/chip is only slightly crossing the antenna, it doesn’t read.
Being careful with that positioning, I was able to get consistently good reads (but often only after a couple of hand/antenna repositions).
I presume that if you hold the position that you got a good read in, that you would be able to get a good write in that same position.