Garageband Plugin not showing UI

Yesterday I installed a plugin instrument into Garageband. The instrument has an extensive UI with a lot of controls. I was able to click on the name in the track and see this interface.

Today, I did exactly the same thing as yesterday but now instead of the interface I'm expecting, all I see is a black box with the name of the plugin and no controls.

I don't understand how this could have happened. I did nothing other than close Garageband and open it again. Help?