Why is the availability of the Plex Mix option so inconsistent?

Is anyone able to explain why the Plex Mix option is missing from random songs in my library so I can fix or change whatever is necessary? I have a large music library and have changed the Schedule Tasks window to allow 14 hours each night hoping that would help but it doesn’t seem to be. Some artists don’t have the option to Plex Mix any of their tracks while some others only have a few. I’m running Plex Media Server on Windows 10 and have tried using the Web (Firefox, Chrome and I.E.), Android and Windows 10 clients all with the same results.

Please, if anyone can share any tips or know why this is happening, that would be greatly appreciated.

Thank you,

I have this too ! over 90 GB of music and yet still some songs don’t have plex mix option - also as of the latest upgrade plex mix DOES NOT work via the web player