Please help, I have tried everything

I have added an mp4 video to Featured Video Plus and it shows the preview in the editor fine, but when you load the page it just shows grey.

I have tried this with and without a featured image, on several different browsers, and made sure that autoplay settings weren’t the culprit. I have also tried to use the same video file in different formats and file sizes, but to no avail.

Any ideas would be appreciated. Thanks.