[AlanHK]

I have epubs from various sources and clean them up and convert to azw to load on my Kindle.

I also set some metadata, but in a few files found these were not working correctly. Every time I saved it in Sigil it would change them: e.g.:

<dc:identifier opf:scheme="AMAZON">
became
<dc:identifier scheme="AMAZON">

and did not work properly (was ignored by kindlegen).

Also a swarm of errors turned up in epubcheck.

Eventually I found that the culprit was a metadata tag.
These files had:

<metadata xmlns:dc="http://purl.org/dc/elements/1.1/">

The well-behaved epubs had :

<metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">

So I assume the latter part was needed to define the tags properly.

Adding that fixed the problem; metadata now works.

I don't know the cause of the omission; but perhaps this will be useful to others.