I went through the exact same thing yesterday. Sound but no video. I was working with Format Factory converting an AVI file to MP4. I finally got it to work.

OK I was searching for info in this forum on MP4 and found this comment by Steve Ledger:

When you convert your video clips to MP4, select 'Setting' and always choose AVC(H264) codec.
DivX is the default selection in FF, which isn't supported.
H264 is the commonly used streaming format (think YouTube/Vimeo etc) as it was basically developed for broadcast.