Apple Music Downloads

My iPhone has presented a problem with Apple Music.

I have created a Playlist in Apple Music on my iMac. (Apple Music v. 1.0.6.10/macOS 10.15.6)

The playlist has appeared in Apple Music on my iPhone. Having completed some edits to playlist order on the iMac, the playlist on the iPhone (8 Plus) has updated. (iOS 13.6)

When I attempt to download the tracks to my iPhone, the spinning circle begins spinning, but the tracks do not download; has 'spun' for hours/days with no change.

All of the tracks in the playlist originate from CD, added to iTunes on my MacPro; the playlist having been created using Apple Music on my iMac appears on my MacPro in iTunes. (iTunes 12.8.2.3/macOS 10.11.6)

If I attempt to drag & drop the Playlist to my iPhone whilst connected to the MacPro it reminds me that iCloud Music Library is enabled for this device, and some tracks (all in fact) cannot be added to the device.

I've proceeded to remove the Playlist (from the MacPro using iTunes) and recreate the same Playlist (on the MacPro using iTunes). Update iCloud Music Library, and the playlist appears on the iMac/Apple Music and the iPhone/Apple Music.

If I ask the iPhone/Apple Music to download the playlist it proceeds too complete some of the tracks, but hangs on the majority.

I have turned iCloud Music Library off on the MacPro/iTunes, quit the iTunes app, reopened the iTunes app, and re-enabled iCloud Music Library. I'm waiting for that Activity to complete before trying anything else.

It is not feasible that running iTunes alongside Apple Music is the reason for the problem, because the Playlist has appeared each time I created it, updated each time I edit it, and disappeared each time I deleted it.

I have an Apple Music Subscription, having previously chosen iTunes Match to make my iTunes Library available.

My iMac/Apple Music allows me to download the playlist contents; all tracks are downloaded and in my User/Music/iTunes folder as appropriate. On my iPhone, it has not succeeded beyond 20% of the playlist.

I have signed out of Apple Music, and iCloud, reset the iPhone. No change.

I have backed up to iCloud & my iMac, and am beginning a Restore.

What reasons do Apple Music have to block downloads on a device? What criteria would legitimately prevent that activity?