How do you write to a tag that is already stuck to a surface?
Exactly as you would write a loose tag, provided the surface still allows a reliable read. Writing needs slightly more energy than reading and needs it held for longer, so a tag that reads marginally in place may refuse a write at the same position.
The operation is unchanged
There is no special mode for writing a tag in place. The phone powers it, sends the write, and the tag stores it, exactly as on a desk.
What changes is the physical situation, and every difficulty comes from there rather than from the operation.
Why writing is harder than reading
Two reasons, and together they explain a frustrating and common experience.
Writing draws more current, because committing data to non volatile memory costs more than reading it back. A tag sitting near its coupling threshold may have enough energy to answer and not enough to write.
And writing takes longer, so the phone has to be held still for more of it. A brief tap that reads successfully can fail a write purely on duration.
The result is a tag that reads every time and refuses to be written, which looks like a locked tag and is not.
When the surface is the problem
If the tag is on metal, it will most likely do nothing at all, read or write, and the answer is a tag designed for metal rather than a different technique.
If the tag is on a curved surface, the coil may be deformed enough to detune it. That is permanent damage rather than a positioning problem.
If the tag is behind a thick cover, the extra distance may have consumed the margin that made writing possible.
The test that separates these from a genuine tag fault is to write a second tag from the same batch in your hand. If that works, the tag on the wall is fine and its situation is not.
Practical technique
Remove the phone case before trying. It is the single most effective change and it costs nothing.
Find the antenna deliberately rather than centring the phone on the tag, since the two rarely coincide.
Press and hold rather than tapping. Writing is not instantaneous and a confident, still contact of a couple of seconds succeeds where a quick touch fails.
And if it still refuses, write a replacement tag in your hand and apply it. The tag costs cents, and time spent wrestling with one in an awkward position costs more than the tag and the adhesive together.
Plan the other way round
Where a deployment can be sequenced either way, write before applying. Writing on a bench with the phone flat and the tag loose succeeds every time, and it removes this entire class of problem.
Frequently asked questions
Why does my tag read but refuse to be written?
Writing needs more energy held for longer, so a tag at the edge of its coupling range can read and fail to write. Remove the case, find the antenna precisely, and hold still for a couple of seconds.
Can I write a tag through a wall or a cover?
If the material is not conductive and the total distance still allows a solid read, yes. Thickness is the constraint rather than the material, for anything that is not metal.
Is it safe to write a tag repeatedly while trying?
Yes. Failed attempts do not damage a tag, and write cycle limits are far beyond anything you will reach by retrying. The only real risk is a write interrupted partway, which reformatting fixes.
Should I write tags before or after applying them?
Before, wherever the process allows it. Writing on a bench is reliable, writing on a wall is not, and the sequence is usually a choice rather than a constraint.