I got my NExT installed in the usual spot between forefinger and thumb three weeks ago. Since then I have not been able to read it with my phone (using tagwriter and NFC Tools) or with the ProxMark3. I haven’t had any swelling that I could tell except for the first day.
The swelling can be hard to notice, but definitely has a big affect on coupling with the implants
Make sure you check out
For tips on where to read the chip on your specific phone
Also make sure to play around with the xFD’s that came with the implant to learn placement
Also also, you can try the lf tune and hf tune commands on the PM3 to further refine the reading location
Also also also, just a quick warning, do be careful if you try to write to the LF side of the implant before being sure of coupling, it is possible to brick it.
Also^4, welcome to the community!
tl;dr: keep waiting and keep trying 
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on your proxmark do hf 14a reader -@ this will continuously poll for iso14443A UIDs (like the one in your NExT)
use the bottom of the proxmark on the edge furthest away from the red ring low frequency antenna as your probe, you should be holding the proxmark perpendicular to the implant so it intersects like a +, once you’re getting consistent UID output from this command do hf 14a info & hf mfu info to get the full run down on your ntag216.
as for low frequency, the T5577 in the NExT comes pre-programmed as an em410x ID, we can do a similar command lf em 410x reader -@ same deal, continuous scan mode, this time using the red circular LF antenna as your probe you’ll want to again try and intersect the antenna with the implant to make a weird bent cross -)- shape. once you’re getting consistent outputs from that command, lf t55 detect
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Hi there. I am running into a similar problem. The HF chip in my NExT won’t read, even with a Proxmark 3 Easy. Using your continuous poll method, I finally did get this read:
[usb] pm3 --> hf 14a reader -@
[=] Press <Enter> to exit
[#] BCC0 incorrect, got 0x00, expected 0x01
[#] Aborting
[=] Card doesn't support standard iso14443-3 anticollision
[+] ATQA: 00 00
[#] Multiple tags detected. Collision after Bit 1
[#] BCC0 incorrect, got 0x00, expected 0x01
[#] Aborting
I’m not really sure where to go from here. I have tried Amal’s method of zeroing out blocks 4-15 with no luck. Swelling should no longer be an issue. I was able to write to it just fine for weeks and now suddenly I’m getting the Android error: Invalid Tag. Would love your advice.
and this is on a NExT? curious.
do you get that error as consistent output with your hf 14a reader?
try
hf 14a conf --bcc ignore
then redo the hf 14a reader -@
Yes, on my NExT. I ordered and installed about three weeks ago. I’ve followed your above advice and turned off all of them. I just keep getting errors. I set them all back to standard and then turned off bcc again.
[usb] pm3 --> hf 14a reader -@
[=] Press <Enter> to exit
[#] Multiple tags detected. Collision after Bit 1
[=] Card doesn't support standard iso14443-3 anticollision
[+] ATQA: 00 00