How do I troubleshoot iTunes failing to add songs to my iCloud Music Library?

I want to add a couple songs to an iCloud Music Library playlist (in iTunes 12.6.1.25 on OS X 10.10.5), but the songs aren't in my iCloud Music Library:

If I right-click on the songs and select "Add to iCloud Music Library", the "spinner" appears in the upper-right corner of the iTunes window and shortly afterwards disappears. The songs continue to have the cloud icon with the slash through it:

I guess it's failing without displaying an error message. I get the same message (as above) when I again try to add the songs to the playlist.

I tried forcing an update as described in https://apple.stackexchange.com/a/197896/94, and I also tried signing out and back in and (again) forcing an update.

How can I get more information on why these songs are failing to sync to my iCloud Music Library?

The icon you're getting (i.e. a single Cloud with a diagonal slash from top left to bottom right) means your song isn’t eligible to be uploaded to your iCloud library.

For example, this can happen if the song file is larger than 200 MB, or if it's encoded at 96 Kbps or less.

For more information you can refer to:

• iTunes for Mac: Symbols used in iTunes
• About iCloud Music Library icons and status

To quote Apple from the second link above:

This icon appears next items that can't be added to iCloud Music Library. A song might be ineligible if it wasn't purchased from the iTunes Store or was purchased using a different Apple ID, or if the song file is larger than 200 MB, is longer than two hours, or was encoded at 96 Kbps or less.