Why aren't my tags populated after importing?

I've tried importing songs from an iTunes directory structure. The original iTunes library files were corrupted, so I'm relying on the tags on the files themselves.
However, after importing the iTunes folder (and managing into a second folder to keep my original file structure intact) a huge portion of my tracks are identified as Unknown Artist, and no tag info is populated into Songbird despite verifying that tag information does exist (using EasyTag).

If I use EasyTag on the managed files after import, Songbird doesn't appear to pick up the information, but if I use EasyTag to "fix" the tags before importing, the tracks show up correctly.

Are there limitations to the types of tags Songbird can correctly parse?
Is there a way to force Songbird to "refresh" its records of the metadata on the files it's already managing?