Someone mentioned on the weekend seeing differing behaviour between playlists that had more than just the title field populated, and ones that didn't. I know all mine have both title and source populated (I made a macro to do that for me). I can click on the PLAY link of the player root and all my songs will appear in Winamp (~5925).

You may want to look at your playlist tags to see if there's some such pattern to them.

Bruno