Shuffle does not include all my music

[pcrossl1]

The selection is very biased towards certain artists. It is certainly not a true random of my entire collection. Some artists are missed entirely.
I cannot see any potential reason for the bias like date of files, artist name, track name
Anybody else noticed this or have ideas how to fix.
I am running a Cambridge Audio Magic 6 connected to my pc running aset uPnP. All my music files are flac files stored in diectories by artist name in the music folder on my pc

[Spoon]

Re: Shuffle does not include all my music

Shuffle uses Rating in the ID tags also.