The tag is not locked but still refuses writing
Four causes with nothing to do with protection. The message is larger than the free space. The tag moved before the write completed. The chip is not an NFC Forum tag type. Or the tag is unformatted and the tool will not format it.
The message does not fit
A message larger than the free space is rejected rather than truncated. The tag keeps its previous content and the tool reports a failure.
Read the tag for its free space and compare against the content you are writing, including everything appended to it. Tracking parameters on an address are regularly longer than the address.
The advertised capacity is the wrong number to compare against, since the fixed structure and the record headers take their share first.
This is the most common non protection refusal and the easiest to confirm.
The tag moved
A write is an exchange that takes time, and moving the phone partway abandons it.
The symptom is a refusal that happens sometimes and succeeds when you hold still, which distinguishes it from every other cause here.
Rest the phone on the tag rather than holding it, and do not move it until the tool confirms. Writing takes longer than reading, so a gesture adequate for reading can be too brief for writing.
This matters most on larger content, where the window is longest.
Not a tag type the phone writes
If a reading tool reports a technology but no NFC Forum tag type, phones will not write it as a tag and generally will not read it either.
This is not protection and it is not repairable. It is a different product from the one you expected.
It arrives from listings that say NFC compatible without naming a tag type, and from very cheap bulk lots.
Check several tags from the delivery before concluding anything about the batch.
Unformatted, and the tool will not format
A blank tag with no NDEF structure needs formatting before it accepts NDEF content, and not every tool offers to do it.
The refusal then looks like a protection error even though the tag is entirely writable.
Use a tool that formats, format the tag, and write again. Formatting is a normal preparation step rather than a repair.
If formatting itself fails, go back to the tag type question, because a chip that will not take an NDEF structure is usually not a Forum tag type.
Frequently asked questions
How do I tell a size problem from a protection problem?
Read the tag. It reports both the free space and the writability state, which separates the two immediately.
Why does it sometimes work?
Intermittent success points at the tag moving during the write. A refusal from protection or from size happens every time.
Does a failed write leave the tag half written?
A rejected write leaves the previous content intact. An interrupted write is a different case and can leave the tag inconsistent.
Should I format every new tag?
Only if it needs it, and most tools handle it silently. A tag that refuses NDEF content and reports no structure is the case where it is worth doing explicitly.