Videos Not Playing in All Browsers

I am using the Enfold theme in WordPress 4.0 on my Mac. I am self hosting six mp4 videos on my site – two each on three pages- and they all view fine on Safari, but when I switch browsers two of the six are viewable, but four appear as black boxes, don’t play, and there are no durations in the controls. All the videos are configured exactly the same. Also, when viewed on a PC -even in Safari- the same problem occurs, but it’s blank space above the controls instead of black boxes.

I tried embedding the videos from my Vimeo site, and they play on my mac in Safari, but in no other browsers. On the PC, the poster of the embedded Vimeo video appears with no duration, but a blank “flash” logoed image appears when played. Can someone please help me with this issue? Much appreciated.