@TuxGirl:
The problem with these files is that they are all ancient 'PalmDOC' files. Unfortunately, the .prc extension is sometimes used with MOBI files, and sometimes used with PalmDOC files, and both types will load on the Kindle, so it's not easy to tell which type of file it is without examining the contents.
Calibre does not support setting metadata for PalmDOC files, meaning that in fact it is necessary to convert these as you were in the process of doing when I told you it wasn't necessary.
The only way to know that calibre didn't write the metadata is to be running in debug mode. I'm going to talk to Kovid about adding some information in the job summary so that you could discover the problem files there. In the meantime, the simple solution is to note which files didn't have their metadata changed on the Kindle Home screen, then do a MOBI-MOBI convert on those. Somewhat inelegant, but it will work.
G