Are you using a Custom Phone layout and the video has not been set?
Also when adding a video to row background the info is given:
"The placeholder image is used on mobile devices that do not support inline autoplaying videos."
Maybe this is why. I am able to add an .mp4 Video via > Customize > Mobile > Background > Video and also working through the Gridder Row background video.
"and if i use Rowbackround Video i cant use one Video for all content...."
If doing this from the Gridder then you will need to apply it to all Gridder's. I would suggest you try a different .mp4 video without sound as a test & also make sure of the following:
Wordpress, Lay Theme & Laytheme's Addons are all up to date ?
Any custom CSS or JS that may interfere is temporarily removed.
Best wishes RichardAny third-party plugins are de-activated to see if this resolves the issue.