The tag fails behind glass or a screen protector

Plain glass is transparent

Glass is an insulator, so the field passes through it without being cancelled. A tag behind an ordinary pane reads.

What it costs is distance. The pane sits between the tag and the phone, and a shop window is thick enough for that to matter when the tag is small.

So the fix for plain glass is a larger tag rather than a different material. A card sized tag has margin a small sticker does not.

Coated and wired glass

Low emissivity and solar control glazing carries a metallic coating, which behaves like a thin conductive layer and reduces or cancels the field.

Heated glass and some security glass contain wires or a mesh, with the same effect.

These look like ordinary glass. A coating is visible only as a faint tint at an angle, and a mesh is sometimes visible in the right light.

Test rather than assume. Hold the tag against the glass and read through it, then hold the same tag in the air, and compare.

A metal window frame matters too when the tag is near it, even if the glass itself is fine.

Screen protectors are irrelevant

The NFC coil is on the back of a phone, not behind the screen. Anything applied to the front has no bearing on tag reading.

This comes up often enough to be worth stating plainly, because people replace protectors trying to fix a reading problem.

What does matter is anything on the back: a case, a metal mount plate, a card in a wallet case, a grip with metal parts.

Making a shop window work

Use the largest tag the design allows, since glass thickness is a fixed cost you cannot remove.

Put the tag at eye or hand height, on the inside, and print a clear target on the outside facing sticker so people know where to hold the phone.

Keep it away from the frame and from any heating elements in the glass.

And test with the oldest phone you can find, through the actual glass, before printing anything. A tag that works on the bench and fails through six millimetres of glazing is a common and avoidable disappointment.