Song mixes aren’t playing all songs, album mixes aren’t playing all albums, not even smart playlists are playing all songs, unless I tap play on the first song of the collection.
You have no provided any logs or details so I can’t tell you more than what is written in the documentation …
The mix won’t play the last 15% most played of the songs / albums so the playlist will always mostly be size - 15%.
Then apply the things described again in the docs, then try to ensure proper non repetitive things during the mix. So if you have a lot more songs from 1 artist than from the rest at the end of the playlist things that could not be properly reparted will be left for the end.
If you just want a random use the random button and take time to read the doc about the mixes.