Nothing happens when I tap the tag: where to start
Five checks, in this order: is NFC on, is the case in the way, where is the phone antenna, does another tag read on this phone, does this tag read on another phone. Most faults are found by the third, and none of the five needs any equipment.
Five checks in order
One. Is NFC switched on? On Android it is a setting that can be off without any indication. On an iPhone the hardware is always available on supported models and the behaviour differs by model, which the phone side guides cover.
Two. Remove the case. Ordinary cases cost a little distance. A case with a metal plate for a magnetic car mount, or a wallet case holding a bank card, stops reading outright.
Three. Find the antenna. The coil in a phone is small and its position varies by model. Move the phone slowly across the tag rather than holding it in one place.
Four. Try another tag on this phone. If nothing reads, the phone is the variable and the tag was never the problem.
Five. Try this tag on another phone. If it reads there, the first phone or the gesture is the variable.
Why the order matters
Each check is cheaper than the one after it, and each eliminates a family of causes rather than a single one.
Checking the setting takes five seconds and rules out the most common cause of all. Sourcing a second phone takes longer and is worth doing only once the free checks have failed.
Doing them out of order is how people conclude a tag is dead while NFC is switched off.
Change one thing at a time. Removing the case and swapping the tag together tells you a fault went away and nothing about which change fixed it.
If all five pass
If another tag reads on this phone and this tag reads on another phone, then both are working and the failure is in the combination. That points at margin: a small tag, a weak phone, a difficult surface, or a gesture too brief.
If the tag reads nowhere, it is either physically damaged or it is not a chip phones can read. Both are covered separately, and both are decided by reading it with a tool that reports the tag type.
If the tag reads and the phone reacts but nothing opens, the tag is fine and the content is the problem, unless a notification is waiting to be tapped, as happens on an iPhone and, for a web link, from Android 17. That is a different branch entirely and it is the most common outcome of all.
Faults on an installed tag
Take it off and test it on a bench if you can. An installed tag has a surface behind it, possibly metal, and often a neighbour, and each of those changes the result.
If you cannot remove it, check what is behind and beside it before suspecting the tag. A tag applied over an older one that was never removed is a common and confusing fault.
Check whether anything changed around it. A metal shelf installed nearby, a new sign screwed over it, a wireless charger placed on the other side of a desk: all of these turn a working tag into an intermittent one without touching it.
Frequently asked questions
How long should I hold the phone against the tag?
A second is usually enough, and larger content needs longer. A brief brush is the most common reason a working tag appears to fail.
Does it matter where I touch the tag?
The overlap between the two antennas is what counts, so aim the phone's coil at the tag rather than the phone's centre. Moving slowly across the tag finds it.
Could the tag be empty rather than broken?
Yes, and it is common. An empty tag is detected and gives the phone nothing to do, which looks identical to a fault until you read it.
Should I try rebooting the phone?
It is worth one attempt after the five checks, since it clears a stuck state on some devices. If it works only until the next failure, that is its own symptom.