Your mini controller window could simply be off the edges of the screen somewhere. It remembers the last position. 

If it were Windows I could help. 

Hi, that’s a great thought.  However, using Mission Control on the Mac does not show the mini controller window.  Thanks again for responding!

I have a similar issue; the mini controller shows, but it does not stay on top of open windows. As soon as I click another window the controller goes to the back.  I can’t find a tick box for ‘stay on top’

It’s an intermittent problem - the mini controller was staying on top last week, but in that scenario, I had to keep the main Sonos window open (not minimised) in order for it to work.

As Mac users, we also do not get the mini controls on the Mac taskbar for open apps. :(

Hello, 
It’s been 2 days that I can’t display the mini controller window.. Clicking on the music cover on the main window “Sonos controller” or doing “Mini controller” shortcut (alt + cmd + 2) don’t solve anything.
It just “disappears”.

But I just find out that moving the main window from Display 1 or 2 to the other (I have an external display plugged in with extended display option) make it appearing.

Maybe it could be a way to deal with that annoying bug until it’s solves by a Sonos app update.