Another tag nearby breaks the read
A second tag inside the phone field competes for energy and answers alongside the first, so the phone cannot resolve which one you meant. The tag causing it is usually the one you cannot see: an old tag left underneath, or a card behind the surface.
What happens
A reader wakes everything in its field. Two tags respond to the same enquiry, and a phone reading a tag in the ordinary way is not built to present two results.
They also load each other. Two coils close together shift each other's tuning, so both collect less than they would alone.
The visible result is a read that works sometimes, depending on exactly where the phone sits, which is the signature of this fault.
The tag you cannot see
An old tag left in place and covered by the new one. This happens during refreshes, when someone applies over rather than removing, and it is the commonest version of this fault.
A tag on the other side of a thin panel, door or partition.
A contactless card in a wallet, a badge holder, or a pocket on the other side of the surface.
An inlay already embedded in the object, in a card, a badge or a piece of packaging that was never meant to carry two.
Confirming it
Peel the tag off and read it in the air. If it reads perfectly alone and badly in place, something at that location is involved, and a neighbour is one of two candidates along with the surface.
Look under the tag once it is off. An old label underneath settles the question immediately.
Move the tag a hand's width and retest. If it works there and not in the original spot, and the surface is the same material, a neighbour is the likely cause.
And read what actually comes back. A neighbour answering instead of your tag looks like success unless you check the content.
Fixing it
Remove the old tag rather than covering it. During any replacement, take the previous one off, which costs a minute and prevents a fault that is genuinely hard to diagnose later.
Separate tags in the plane. Leave at least the width of a tag between them, and more where the tags are small.
Give each tag a printed target so people aim at one rather than at a region containing two.
And where several options belong in one place, use one tag pointing at a page that lists them. It is more reliable than a cramped row and it never misreads.
Frequently asked questions
Do two tags damage each other?
No. They interfere while a reader is present and do nothing to each other otherwise, so separating them restores normal behaviour.
How far apart should tags be?
At least the width of a tag, and more for small tags. The separation that matters is set by the phone's field rather than by the tags.
Can a phone read two tags at once?
Dedicated readers can enumerate several, and a phone reading a tag normally is not built to present two results.
Why does it work sometimes?
Because which tag wins depends on millimetres of position. Intermittent behaviour that changes with placement is the signature of this fault.