Black bars on video objects in knowledge check slides

When I add a video object onto knowledge check slides, black bars often occur on either top or sides of the object, when I preview it. See example on the picture below, here the black bars occur on top and bottom: I can manually adjust the object size and sometimes hit just the right dimensions, so the bars do not occur – but there must be an easier way to do this? “Maintain aspect ratio” is checked (and grayed out) […]