Cannot write tags to some M4a files

Using MP3Tag to clean-up my library and found some M4a files that MP3tag could not read or write tags for. Did a search of the forumand it looks like this may have occurred previously but with m4v video files. As an FYI, I can read / write tags in itunes v10.1. I thought maybe the tags were corruptso I tried to use ID3Killl to clear all the tags but that didn't do anything.

Also tried another editor, TagScanner v5.1, and it cannot read / write tags for the files either. So this may not be a "bug" in MP3Tag.

Anyone got any ideas?

are you sure these don't have DRM?

All tracks on the album are edittable except for one. The tracks were DL'd from iTunes recently so they should have any DRM as iTunes has removed / stopped using it haven't they?

yes, that's my understanding (about itunes and DRM). No other clues on my end.