Display wants to be somewhere. If it is not open in a tab somewhere, then it will appear in the side bar down in the same area with "action window". But, as long as it is open in some tab, somewhere, it will not appear in the side bar.
I'm guessing that you got a new tab open, but you did not click on playing now and your zone that you are playing audio in. You need to have the display for your currently playing zone "viewable" in a tab somewhere. What I mean by viewable is just that it's showing when the tab is open. Then you can switch to another tab, which hides it, but it's still "showing" as far as MC is concerned. I hope this make sense.
Brian.