Andrea,
The "file information was retrieved for 0 of 1 files" indicates that you submitted 1 file to YADB for tag information, and the "0" indicates the number of files (out of the total number of files submitted, which in your case is 1) for which tag info was retrieved. Thus, since 0 of the files were updated with tag info retrieved from YADB (no info was retrieved because YADB either doesn't have info for that track or the search "mechanism" is flawed), you saw no change in the file properties window for that track.
If you still want a better understanding, try selecting a group of (what you think) are commonly owned tracks and try submitting them (even if you already have the tag info for the files, or maybe change some of the tag info in some of the files you'll submit to erroneous data), and then view the results message. You will probably see something like "file information was retrived for 17 of 20 files".
Wobbley