I am using OS X but I'm not sure if this is an OS X specific problem, so I thought I'd post here.
I have had some trouble with files not appearing in the Media Library or receiving a 'no tracks found' error, despite these files being playable when accessed via the folder structure. I have discovered the 3 situations that trigger this problem for me.
1. As discussed in this thread: http://ps3mediaserver.org/forum/viewtopic.php?f=6&t=1747&p=7481&hilit=apostrophe#p7481 , apostrophes in directory names will cause the 'no tracks found' error. This is because the apostrophe causes an SQL error when searching the database. This is shown in the Traces panel.
2. Unknown extensions in directories. I had some directories with '.wpk' and '.mid' files. During scanning, these directories caused the scan to fail.
3. Empty directories. If there are no files in the directories being scanned the process will fail.
My suggestion to everyone having a problem with the Media Library not showing files is to run a scan, and take a close look at the debug.log file to see where the scan stopped. On my system, there was no error message, it simply said it was scanning a directory, but there was no further output.
I hope this is helpful to at least one person out there.