Why is one .mp4 video compatible and a second .mp4 video incompatible?

Why is one .mp4 video compatible and a second .mp4 video incompatible?

Normal reasons include: use of non-standard or incompatible encode settings, workflows that modify standard file and/or container data, and/or file corruption.

Can you please post the steps to find out what it is?

I can possibly point you in the proper direction.

The compatible video (plays fine) info says: Dimension: 720-480, Codecs: AAC, MPEG-4 Video (.mp4 file)

The in-compatible video (does not play) info says: Dimension: 224-400, Codecs: H.264, AAC (.mp4 file)

Unfortunately, you will need to dig deeper to properly analyze the files. Basically, MPEG-4 files are further defined by their Profile and Level. These values further determine which features are "standards" compatible. (E.g., see charts MPEG-4, Part 2 and H.264/MPEG-4 AVC) To be QT compatible, MPEG-4 files must remain "standards" compliant. Third-party and/or proprietary encoders may allow the hybridization (or mixing) of features using non-standard Profiles and Levels. In addition, the data must be encapsulated in a properly configured container (as recognized by a specific security update/QT Player combination) and the particular QT player in use must be programmed to recognize/use the data compression/file container combination.

Normally, my first action is to try and play a file in multiple player applications—e.g., QT X, QT 7, VLC, Player, NicePlayer, etc. If it plays in some but not others, I would usually assume that the file and container are not corrupted. If it won't open and play in any player, I would tend to assume the file itself is bad. If the file opens/plays in any player, I would next analyze the encode settings and features used and cross reference them with the Profile/Level charts previously linked above. For instance, QT players don't seem to support the highest Profile/Level combinations even when "standards" compliant. Many third-party and/or proprietary MPEG-4 AVC encoders improve output quality at lower Profile/Level settings through the use of advanced features normally only associated with higher, more advanced Profile/Level combinations. Thus, it is sometimes necessary to know how the file was originally created. In other cases, it is necessary to know what workflows were applied to the problem files. For instance, copying large (> 4 GB) files to storage devices having operating systems limited to 4 GB will corrupt the copied files making them unplayable.