Check the video files with this player: VLC - (click on link) _ (choose nearest mirror site) before adding to iTunes. You can preview the file before it finishes downlding. To prevent a preview file being created in the incomplete folder, go to LW's Library window, select incomplete folder, select the video & press launch. Alternatively, open VLC & direct it to your incomplete folder.

Some video files can be encoded with a variety of audio formats. Some of which iTunes might not be able to understand for video files.