Why do some Android phones read tags others cannot?
Three causes, and only one of them is exotic. Antenna position explains most reports, field strength explains the next largest share, and support for particular tag technologies in the controller explains the remainder, which is the one genuine hardware difference among them.
Antenna position
The coil is small and its location varies by manufacturer and model. A person presenting the middle of the phone succeeds on one device and fails on another with the same tag.
This is the largest single cause and it is not a hardware defect. The phone reads perfectly at the right spot.
It is also invisible to whoever is testing, because they have learned where their own phone reads without noticing they learned it.
Field strength
The field a phone produces varies between models, and a weaker one gives less working distance with every tag.
The effect compounds with tag size. A card sized tag forgives a weak reader, and a small tag near its limit fails on the same device.
So a deployment validated on a recent flagship can disappoint in public, because the flagship was hiding a margin problem rather than proving there was none.
Cases and accessories subtract from whatever margin remains.
Technology support
This is the genuine hardware difference. Android exposes the underlying tag technologies individually, and support for them is not uniform across controllers.
Type 2 is read by essentially every device with NFC hardware, which is why it is the safe answer for anything public.
Type 5, built on the vicinity card standard, is supported on many recent devices and not uniformly across the installed base.
MIFARE Classic is the well documented case: it uses a proprietary protocol above the standard radio layer, and Android states that support depends on the device.
Choosing to avoid all three
Specify Type 2, which removes the technology question entirely.
Use the largest tag the object allows, which buys margin against weak readers and imprecise aim at the same time.
Print a target so people aim rather than wave, which addresses the position problem without knowing anything about their phone.
And test on the worst device you can find rather than the best. A deployment that satisfies the weakest phone in your audience satisfies everything above it.
Frequently asked questions
Is one manufacturer better at NFC?
There is real variation between models and it does not follow brand lines cleanly. Test the devices that matter to you.
Why does my tag work on a new phone only?
Usually field strength or antenna position rather than a capability difference. Try a larger tag and a slower sweep on the older device.
Which tag type works everywhere?
Type 2. It is read by essentially every phone with NFC hardware, which is why it is the default for public deployments.
Is MIFARE Classic readable on Android?
It depends on the device, since it uses a proprietary layer above the standard radio. That is why it is a poor choice for anything public.