When is a phone actually listening for tags?
Not continuously, and not simply whenever NFC is switched on. A phone looks for tags in short bursts, under conditions that it decides for itself, and that is what explains the first tap which does nothing and the second which works a moment later.
It looks in bursts
Producing a field costs power, so a phone does not hold one continuously. It energises the field briefly, listens for an answer, and stops.
That cycle repeats while the phone considers itself in a state where a tag is plausible.
So presenting a tag is not presenting it to a permanently open ear. It is presenting it and waiting for the next look.
The interval is short enough to feel immediate and long enough that a genuinely instantaneous brush can fall between two of them.
The conditions it applies
Whether NFC is enabled at all, which is the obvious one.
Whether the screen is on and the device unlocked, which on Android is normally required for tag dispatch and varies between manufacturers.
Whether a power saving mode is restricting background activity, which some devices extend to NFC.
And on iPhone, whether the model supports background reading or an application has to be open. Background reading is available from the iPhone XS and XR onwards.
Take NFC Tools. On iOS it requires an iPhone 7 or later running iOS 15.6 or later, and the app must be launched manually. Background reading is available from the iPhone XS and XR onwards.
What this explains
The first tap that does nothing and the second that works. Nothing changed except that the second one coincided with the phone looking.
Why holding still helps more than tapping repeatedly. Holding guarantees you overlap the next look, and tapping gambles on timing twice.
Why a sleeping phone in a pocket never reacts. It has decided a tag is not plausible and stopped looking.
And why the battery cost of leaving NFC enabled is small: the field is on for a fraction of the time rather than continuously.
What to do with it
Rest the phone on the tag rather than tapping it. One deliberate contact of about a second beats three quick ones.
Tell visitors to hold rather than tap, in those words, because the word tap invites exactly the gesture that fails.
Wake the phone first when testing, so you are testing the tag rather than the phone's state.
And when diagnosing, repeat the same gesture several times before concluding anything. A single failure tells you very little about a system that looks intermittently by design.
Frequently asked questions
Why did it work the second time?
Because the second attempt coincided with the phone looking. The field is produced in bursts rather than continuously.
Is my phone always emitting?
No. It energises the field briefly and repeatedly rather than holding it on, which is why the battery cost is small.
Does holding longer really help?
Yes, and more than tapping repeatedly. Holding guarantees you overlap the next look instead of gambling on timing.
Does the screen have to be on?
On Android tag dispatch normally requires an awake unlocked device, with variation between manufacturers. On iPhone the model decides.