Movie Textures not working - "Not ready to play" (Unity Pro)

Hi everyone.

I'm currently trying to play full screen movies in my game and the main method for doing this that I've found so far is by using Movie Textures - however I'm having some issues.

So far I've tried several videos and I've only managed to get a .mov file working, other formats either aren't recognized correctly as movies in the project window and the ones that do aren't showing up correctly in the inspector.

Here's how all of my MP4 files look, which should be a supported format.

As you can see, there's no preview and it says it's "Not ready to play yet". The dimensions listed here (16x16) are also incorrect. These files all open and play correctly in QuickTime and VLC, so I don't think there's anything wrong with them - the file shown here is actually a sample mp4 I downloaded from the QuickTime site as a safe test.

Does anyone have any ideas about what might be the problem here? Are there any other methods of playing movies that I could use instead of Movie Textures? Any help/advice would be much appreciated!