Quick Time Player unable to play MP4 files

Hello Carol

Here is the most common reason why Quick Time won't play your MP4 files:

Quick Time will only play MP4 files that have been encoded to meet Joint Video Team (JVT) standards. It will not play files encoded using proprietary codecs, files encoded employing hybrid features/settings, or files containing content other than MPEG-4 (part 2/10) video and/or MPEG-4 (AAC) audio.

You basically have three options :

1. Use a third-party player the supports the MP4 file in its current form (e.g., you might try VLC),
2. Use a third-party app to convert the unsupported content to a supported format (e.g., you might try HandBrake), or
3. You (or someone else) can examine the file to determine the specific nature of your problem and either fix or avoid the problem thereafter.