First check in your AE project that you have a transparent background by toggling the checkerboard switch in the comp window. If you get the checkerboard pattern, then the problem must lie with your render, if you don’t see the checkerboard, then you have a layer in your project that needs to be removed.
The next thing to check is that when you rendered your movie you set the output module -> Video Output -> Channels to RGB + Alpha.
Johnny Cuevas, Editor
Thinkck.com
“I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.”
—THOMAS EDISON on inventing the light bulb.