When selecting the video file on your slide and going to the Playback tab: is the Play Full Screen box checked? I think in order to make triggers work, it should be unchecked.
If you want the video to play fullscreen you can always adjust the size manually, without checking the Play Full Screen box.

Hi Judith, that has worked, thank you VERY much, I've only tested one object but, hopefully, I can add many, throughout the playback