There's a rare instance when videos may not play using the following scenario:
- Virtual Machine (VM) such as Parallels or VM Ware
- Internet Explorer 11
- MP4 videos
This problem is most likely related to the fact that IE11 doesn't seem to support mp4 videos currently, all previous versions do.
Until IE11 updates to support MP4 videos in a VM, here's a workaround:
Press F12 > click the Emulation tab > change the Document Mode to 'IE10 (Emulate IE10)'.