G Mixes is meant to offer you a never ending channel that plays songs that go well together, like a personal radio station.
This means that if most of the music in the library is of just a few genres then the number of mixes will be limited.
To help Genius see what music you have, make sure all the track information is as accurate as possible (Artist, Track Name, Genre), then update Genius (Store menu>Update Genius)
This may help Genius find more compatible music.
And if you want a genius list you can control and edit, then you can generate a song-specific genius playlist, Right click a song then click Start Genius, This playlist displays in “Genius” on the source bar left where you can limit the number of songs in the playlist, Refresh the playlist, delete track from it and even save it.
Hope this helps.