On full-screen mode of TM, "Show Now Playing Assistant" command doesn't work either via context menu command, hotkey, or on-click button.
 I see it works and opens the popup window but just doesn't take focus from the main window. Same when running "Show equalizer & DSP" command. Many apps (like web browsers or office apps) support opening popups on full-screen mode. Hope you support it but if you can't, temporarily turning off full-screen mode could be an alternative to show the popup window.