My recent attempts to add music on-demand to Google Play Music have yielded nothing - no "Processing music..." or "Added __ of __" messages, just nothing.

Previously I could add music on-demand; and nothing has changed on the machine from which I successfully added music previously, from which I have tried to add music on-demand recently.

What could be hampering my ability to add music on-demand?

WHAT I'VE TRIED:

Right after I started using GPM, I briefly found that I could not add music (on-demand), but the problem went away after a logout/login. This time a logout/login has not helped.

Dragging & dropping or browsing to folders or files to add has made no difference either.

Nor has waiting ridiculously long for GPM to show signs of life after adding music on-demand seemed to work.

Digging deeper, I read a related Google Play Help article and followed its suggestions...

• ran the Google Play Music Manager troubleshooter => no errors or warnings
• double checked my available storage => 8 GB free
• double checked supported file types => MP3 is still supported (of course)

..., but the problem remains.

UPDATE:

I found that if I configure GPM to automatically upload music added to specific folders, it strangely does add automatically what it will not add on-demand.