Mac Mini won't shut down via screen sharing app

I have a headless Mac Mini which I run via the screen sharing app on my MacBook PRO running High Sierra (I run legacy software). No matter what I do I can't shut down the Mac Mini via screen sharing. I select the options in various ways but the mini keeps running and I have to turn it off via the power button. Any idea why the OS interface on the mini is ignoring the shutdown command? I go to shut down and it says other users are active, so I enter the primary admin username and password to shut without saving and it just goes to the startup screen. I go to the shut down icon top right and it repeats the 'active users' dialog and never actually shuts down, returning to the startup screen again.