I have the same problem of tracks not available to play. I was using the Web app today when it occurred. I looked up an album, Herbie Hancock's Possibilities, and I couldn't play When Love Comes to town.
This was the first time that I had looked up the album or song, so it couldn't have been an issue with cache. And it wasn't a local file.
I have had this occur in a number of locations through the system, but this was the first time I've detailed exactly where and when it showed up.
@Dbass1979 - Just had a look into the catalogue for you, unfortunately that album is currently showing as unavailable on Spotify in all countries which normally means it has been removed by the record label for some reason.