Media Completes trigger fail

Hello!

The screen capture shows a slide where I am using a rectangle to prevent User clicking on the items at the right. I want the instruction narration ("Vox Instructions" in green in the screen cap) media to complete, then hid or disable the "choices blocker" after the narration is over. I can not get the trigger dialog to accept  the media. It "sees" it and it highights when I arrow over, but when I click, "Vox Instructions" is not selected. "Vox Instructions" resides in the base layer.

I've rebuilt this a couple of times without success. Based on my description and screen capture, can anyone point me to possible causes for this issue?

Thank you in advance