Hello Gabrielle,
Did you test with chrome browser?
If so, It seems an issue of Chrome on mobile. Because I tried to convert video to some formats what supported by major browser (.mp4, .webm, .ogv), they worked normally on Firefox, but Chrome.
So, for temporary solution, please add image for video poster parameter, then image will be displayed instead video in case of browser can not play video.
FYI: We will update masthead module to support add more than one video source for video background image type, so video can be displayed on more browsers and devices. The new version will be released this week.
Best regards,
Thanh Nguyen
Thanh Nguyen
Founder & Lead Developer