Seems the background video was loading but the default image was showing above it and covering the video. Fixed with this:
.metaslider .slides .ms-layer img.msDefaultImage {
z-index: 1;
}
.metaslider .slides .ms-layer video {
z-index: 2;
}
.metaslider .slides .ms-layer .msHtmlOverlay {
z-index: 3;
}