Cloud music library update stuck - new imported music won't sync all of a sudden

The tech I spoke to asked me to update to 11.5.1 (from 11.5) but he didn't have any specific information on whether it would fix the issue (as we know, it didn't). There's no logic I can see that determines what will sync and what won't. Mine seems to stick at about 1000 songs (of 12,500). I found that my latest addition (ripped from a CD) got in to the cloud but songs from Apple didn't. It's interesting that the information at Album level seems to get out to iOS devices (I see Album, Artist and cover artwork) for my entire library, even though I've deleted the online library before syncing - it's just the songs that are stuck "waiting" that are greyed out. One of the techs claimed that Apple retains data on what you've previously synced even if the individual songs themselves have been deleted from the library, which sounds logical if all of the metadata is visible on iOS devices.

In my case, the matching part of the service stopped working entirely sometime on Aug 4th. Until then, similar to your experience, it was working really slowly and matching 2 or 3 tracks a day.

I can see the tracks in the Music app on my iPad but, like many others with the same problem, they are greyed out and can't be played. 164 of these tracks were 5 albums that were previously matched. I wanted to relocate the media files so copied them the a temporary location, deleted them from my library, then reimported them from the temporary location.

My take away from this is that details of my library are in the cloud and that tracks that can be matched are being properly identified. The problem seems to lie with establishing the link between the tracks that can be matched with the applicable media files on Apple's servers.

I can also confirm this. iTunes is able to upload a sample mp3 I found online, as well as an album that wasn't in Apple Music. Curiously enough it seems to also hang on matching songs that were pulled from apple music.

And lastly, I seem to be having success with uploading one song at a time, though it takes about 5 minutes per song...

After weeks of being broken, it suddenly started working again for me as of this morning. I had two CDs that were imported to my library that were showing on my iOS device, but the songs were greyed-out.

Just now, I selected 'Update Cloud Library' from macOS Music-->File-->Library... and within seconds it completed its update. This is the step that had been freezing. Apple must have finally done something on their end.

I am having the exact same issue. Please keep me posted if anyone finds a solution.

I have literally nothing going on Tuesday - if nothing is done by them by then, I can try my hand with Apple Support and take as long as needed to get them to see reason