My tag stopped working once stuck on metal

The failure is total

A conductive surface in an alternating field develops currents that produce an opposing field, cancelling the one the tag needs exactly where the antenna sits.

So the symptom is a dead tag, not a weak one. Moving the phone closer changes nothing, holding it longer changes nothing, and a different phone changes nothing.

This is the most frequent single fault in NFC deployments, and it is also the most predictable. Nothing about the tag was wrong before it was applied.

Confirming it in a minute

Peel the tag off and hold it in the air, by its edge, away from anything. Read it at the same distance with the same phone.

If it reads in the air, the metal is the cause and the tag is intact. If it does not, the tag was damaged during removal or was never working.

Then hold the tag flat against the metal without sticking it, and confirm the failure returns. Two tests, one conclusion, no guesswork.

Watch for metal you did not expect. Foil backed plasterboard, a metal stud, a mirror and a metal framed sign all behave like a metal surface without looking like one.

Three routes, by cost

Move the tag. Free, and the one people try last. A hand's width onto a plastic fascia, a wooden edge or a painted panel with nothing behind it can solve it completely.

Use an on metal tag. These carry a ferrite layer that redirects the field so the metal stops cancelling it. It is a construction rather than a coating, so it has to be bought rather than improvised.

Change the mounting. A tag on a small plastic plate standing off the surface, or on a label holder, removes the problem by removing the contact.

Foam spacers sit outside this list on purpose. They recover part of the loss and leave the tag marginal, which is a poor outcome for anything installed in quantity.

If it reads a little

Then the metal is not the whole story, and this is the useful diagnostic.

A partial read points at a partly conductive surface: a metallised coating, a thin foil layer, a painted surface over a metal mesh. These reduce rather than cancel.

It can also mean the antenna overhangs the metal, with part of the coil off the conductive area. Moving the tag slightly then changes the result dramatically.

Or something else is contributing: a neighbouring tag, a thick case, a small tag at the edge of its margin. Test those separately rather than assuming the metal explains everything.