Playlist folder not showing songs from all child playlists

Question

I have created a playlist folder which contains 2 regular playlists (i.e. not smart playlists). The hierarchy is like this:

Occasions
|
+--- Party mix 1
|
+--- Party mix 2

In iTunes, if I click on the Occasions folder, I can see all of the songs from Party mix 1, but none of the songs from Party mix 2.

I can't see any difference between the 2 playlists, and because it's not a smart playlist there aren't any settings available.

Should a playlist folder automatically include all songs from its child playlists? Any ideas about why it wouldn't be showing songs from both playlists?