I have a PC (running Windows) with a collection of MP3 files I ripped from CD. I installed iTunes (18.104.22.168) and setup iCloud Music Library, and it went through the uploading/matching process on my files.
From my iPhone (iOS 11.0.3, and which has iCloud Music Library enabled), I can see all of my library in the Music App. However I noticed that some files would not play - a popup would appear saying "Item Not Available" and "This item can't be played".
I did a little digging in my library, and I found that I have this problem with files with an iCloud status of Uploaded. I can play files with a status of Matched just fine.
The files with a status of Uploaded are not available on Apple Music or the iTunes store - e.g. Turkish music from the 60s. So they will never become Matched. But, I should still be able to play them as an Apple Music or iTunes Match subscriber - that is supposed to be a selling point of iCloud Music Library.
Is there something wrong with my library (iTunes side or iPhone side) that is preventing these songs from playing? I have fewer than 100,000 songs, so that is not the problem.