Change the title

I have been using your software for years and am very happy with it. Now I have a single file whose title I can't change at all. The file is not write-protected. I try alt+2 manually, with action, copy the file and try the copy - nothing worked. The existing title is retained and cannot be overwritten. Any idea what else may work? Thanks

How do you see that

In MP3tag?
Then check Tools>Options>Tags>Mpeg which tag versions you read, write an delete and also check which tag versions are in the file.
It could be that the file has (empty) APE tags, you read them - but you don't update them.