Video from file - sound, but no picture

Hi there

I have a problem with inserting video file.
I've got an MP4 video I'd like to add, but I can only get the sound working - no picture....

I add the file as per the manual, the player shows up on the prezi, I can click "Play", but I only get the sound working - no picture at all.
I tried it several times, but with the same effect.

Any ideas what's wrong and how to fix it?

Thanks,
Krisztina