MP4 videos added as custom content do not start playing immediately, or at all.
1. Verify H.264 codec:
Verify that your MP4 files were encoded using the H.264 video codec. Free programs such as VLC or MediaInfo can be used to verify the codecs used.
2. Verify moov atom placement:
If you have verified that your video was encoded with H.264, the issue is most likely the fact that the movie atom is not located at the beginning of your video files. For progressive download or streaming, the moov atom (movie atom) will have to be at the beginning of the file. This ensures that the required movie information is downloaded first, enabling playback to start right away. If the moov atom is located at the end of the file, it forces the download of the entire file first before it will start playback. If the file is intended for local playback, then the location of the moov atom will not impact the start time, since the entire file is available for playback right away.
The placement of the moov atom is specified in various software packages through settings such as "progressive download," "fast start," "use streaming mode," or similar options. Software packages such as MP4creator or AtomicParsley enable you to analyze the location of the moov atom in your encoded files.
Unless specified otherwise, the moov atom is normally stored at the end of the file in on-demand content, after all of the information describing the file has been generated. Depending on the type of on-demand delivery method selected—progressive download, streaming, or local playback—the location will need to move either to the end or to the beginning of the file.
Some tools enable relocation of the moov atom to the beginning of the file's structure through post-processing of the compressed MPEG-4 (MP4) file. One such tool is MP4Creator, mentioned earlier, as well as MP4 FastStart. The best way to handle the moov atom location, however, is to set it during the compression and muxing portion of the encoding process. This minimizes the probability of the moov atom inadvertently being placed at the end.
The excerpt is above taken from the following article: Understanding the MPEG-4 movie atom