What flavour phone do you have?
Check out this thread, there may be so little tips or tricks for your specific phone on there.
At the moment, I think it is likely just due to swelling.
But a couple more things you can try
Do you have any LF things you can test with?
Did you get xAC with your order?
Otherwise Grab your HF xFD that came with your order (the non implantable glass tag with 13.56MHz written on the card.)
Make sure your phone is:
On
Unlocked
No cover
NFC turned on
Then, SLOWLY move your xFD around on the back of your phone (top edge on iPhone) to find the “sweet spot” LOCATION and ORIENTATION
Replicate this with your NExT
Good luck
here is the ACR122U Rosco suggested