Why does the same tag read differently on two phones?
Because the reader is half the system and it is the half you do not control. Antenna position, coil size and field strength all vary by model, so the same tag can be effortless on one phone and need a precise touch on another. This is normal rather than a defect.
The antenna is not where you think
Phone NFC coils are small and their position varies by manufacturer and by model. Upper back, centre back, and near the camera cluster are all common.
Someone who holds the middle of their phone against a tag will succeed on one model and fail on another, having done nothing differently. The tag is not the variable.
Finding the position once, for the phones you care about, converts a mysterious intermittent failure into a solved problem. It takes a minute per device.
Not all phones push equally
The field a phone produces varies between models, and a weaker one gives a shorter working distance with every tag.
The effect compounds with tag size. A card sized tag has margin to spare and forgives a weak reader, while a small tag near its limit fails on the same device.
This is why a deployment validated on a recent flagship can disappoint in public. The flagship was hiding the margin problem.
What sits in between
A case adds distance, and a thick or hard case adds enough to matter on small tags. A case with a metal plate for a magnetic mount blocks the exchange outright.
Cards in a phone wallet case sit directly between the coil and the tag, and a contactless bank card there both absorbs field and can be seen by the reader instead.
Popsockets, rings and mounts with metal parts do the same at whatever spot they occupy.
Testing so this does not surprise you
Test with the worst phone in your target population, not the best. For a public deployment that means an old mid range handset with a case on it.
Test with someone who has not seen the tag before, and watch how they present the phone. Their gesture is the real requirement.
Put a visible target on the tag. It converts a vague wave into a deliberate touch, which recovers more reliability than a step up in tag size.
And accept the residual variation rather than chasing it. Some spread across devices is inherent, and the design goal is a tag with enough margin that the spread stays invisible.
Frequently asked questions
Is one phone brand better at NFC?
There is real variation between models, and it does not follow brand lines cleanly enough to be a rule. Test the specific devices that matter to you.
Why does my phone read only at one exact spot?
Because its coil is small and located in one place. Once you know where, the same tag becomes reliable rather than temperamental.
Does a phone case always cause problems?
Ordinary plastic and silicone cases cost a little distance and are usually fine. Metal plates, magnetic mounts and cards in a wallet case are the ones that break reading.
Should I state a compatible phone list?
Better to state the gesture and test broadly. A device list ages badly, and a tag with enough margin works across models without one.