My tag only works sometimes
Intermittent faults have a signature, and identifying what the failure depends on names the cause. Position dependence means aiming or a neighbour. Device dependence means thin margin. Time dependence, working then not, means mechanical damage, which is the one that does not recover.
Find what it depends on
Intermittent does not mean random. Try twenty taps and record what changed between the successes and the failures.
Vary one thing at a time: where the phone is placed, which phone, how long it is held, and whether the tag has been used a lot since it last worked.
The variable that predicts the failure is the diagnosis. Without it you are changing things at random, which is how intermittent faults consume afternoons.
Position dependent
If success depends on exactly where the phone sits, the overlap between the two antennas is marginal.
Causes: a small tag, a phone whose coil is not where you assume, a thick case, or a mounting surface that absorbs part of the field.
Another tag nearby is the one people miss. A second tag within the phone's field competes, and which one answers depends on millimetres. Check for a tag under the one you can see, or a bank card behind the surface.
Fixes: a larger tag, a printed target so people aim, or moving the tag away from its neighbour.
Device dependent
If it always works on one phone and rarely on another, the margin is thin rather than the tag being faulty.
The failing phone has a weaker field, a differently placed coil, or a case. The tag is at the edge of what that combination can do.
This is the fault that hides during testing, because testing usually happens on the best phone available.
Fixes: a larger tag, a better mounting position, and validating on the worst device rather than the best.
Time dependent
If it worked reliably and now works sometimes, and nothing about the position or the phone changed, suspect mechanical damage.
The bond between chip and antenna fatigues with repeated flexing and then makes contact only sometimes. A card in a back pocket is the classic case.
This is the important one, because it does not recover and it gets worse. A tag behaving this way should be replaced rather than investigated further.
The confirmation is simple: an identical tag that has not been flexed, tested in the same position on the same phone, reads normally.
Check also whether something changed around an installed tag. A metal fixture added nearby, a new sign over it, or a second tag applied on top all convert a reliable tag into an intermittent one without touching it.
Frequently asked questions
Could it be the tag going bad slowly?
The chip does not degrade with reading, so a tag that worked and now hesitates is mechanical rather than worn out. Reading consumes nothing.
Why does holding the phone longer help?
Because the exchange needs time, and a larger message needs more of it. A tap too brief to complete the read looks like an intermittent tag.
Does a low battery affect reading?
Power saving modes change device behaviour on some phones, which is a phone side symptom rather than a tag fault. Test with the phone charged to rule it out.
Is it worth trying to fix an intermittent tag?
No, once mechanical damage is likely. A tag costs cents and an intermittent one in service will fail again at the worst moment.