EW 6 Backgrounds
What version and build of EW? The latest version 6 is 6.7.14. If you don't have the latest one you should update to 6.7.14 per the link below.
And did you install and configure the Additional Codec Pack if you are on EW6?
Are your graphics drivers up to date? If not, update them to the latest version for your specific graphics card/chip.
And finally, does your computer meet or exceed the Hardware System Requirements for the resolution of the videos you are trying to use and the displays you are sending them to? If not you will need to update the computer.
My backgrounds for presentation are all black and no videos will play.