Read distance dropped after installation
The surface, almost always. A tag that read well on a bench and badly once mounted is reporting what is behind it rather than a fault. Peel it off, test it in the air, and the comparison names the cause in under a minute.
The one minute test
Peel the tag off, hold it in the air away from anything, and read it at the same distance with the same phone.
If it reads well in the air and badly on the wall, the surface is the cause and the tag is fine. If it reads badly in both, the tag is damaged or was never good.
That single comparison replaces every other guess, and it is worth doing before ordering replacements.
Hold the tag by its edge rather than flat against your palm, since a hand is mostly water and absorbs part of the field.
Surfaces that do this
Metal, obviously and totally. Steel, aluminium, a metal door, a filing cabinet, a machine housing.
Metal you did not know was there. Foil backed plasterboard, a mirror, foil lined insulation, a metal stud behind the plaster, a metal framed poster, foil in packaging.
Wet or dense materials. Damp wood, a concrete wall that has taken up moisture, a container of liquid on the other side.
Electronics. A screen, a wireless charger, or an appliance panel with a metal chassis behind it.
What you cannot see
This is what makes the fault confusing. A painted wall looks like a painted wall, and the foil behind it is invisible.
A useful sign is that several tags in one area all read badly and tags elsewhere are fine. That pattern points at the construction rather than at the tags.
Another is that the same tag works at the edge of a panel and not in the middle, which suggests a frame or a stud.
Moving a tag a hand's width can transform it, and it costs nothing to try before buying anything.
Fixing it
Move the tag if the surface allows. The cheapest fix and the one people try last.
Use an on metal tag where it does not. These carry a ferrite layer that redirects the field so the metal stops cancelling it, which is a construction rather than a coating and cannot be improvised.
Increase the tag size if the loss is partial rather than total, since a larger antenna has more margin to give away.
And test on the real surface before ordering the deployment. A sample stuck to the actual wall for a day answers this before it becomes a hundred tags.
Frequently asked questions
Can I just add foam behind the tag?
It recovers part of the loss and leaves the tag detuned and marginal. An on metal tag addresses the mechanism rather than the distance.
Why did it work on my desk?
Because a wooden desk is transparent to the field and the wall behind your tag may not be. The bench is the wrong test for a mounted tag.
How do I know if there is metal behind the wall?
The tag itself is the detector. If it reads in the air and not on the wall, something conductive is there whether or not you can see it.
Does paint or wallpaper matter?
Ordinary paint and paper do not. Metallic paint and foil backed wallpaper do, and both look like ordinary finishes.