VSE-Background image behind foreground video. Alpha over, alpha under don't work A png image to be used for background.

A video sequence of numbered png files ( which were rendered in Blender with alpha.) is to be the foreground.

Selected backgound png, then video sequence, add effect strip alpha over. No background visible. (Tried every combination I could think of. If use alpha over the background is the only image visible. Use alpha under the foreground is only image visible.  display is sRGB) 

Tried with single .png file as forground. Same results.

I am wondering if the video sequenced pngs have lost the alpha channel. On the video display there is no checkerboard as there was within the 3d scene for the background. (The black part of the video looks solid black on the display rather than checkerboard)
Your PNGs were probably not exported with the alpha channel.

Make sure you export the PNGs as RGBA



To check if  your image has an alpha channel load it in the UV/Image editor.

An image with alpha channel will look like this, with a checker pattern on the alpha:



An image with no alpha channel will not show that pattern, but a solid color:



To make the alpha channel transparent on the VSE, select the strip with the pngs, presss N to make the panel visible, and in the blend type select alpha over.