Handbrake x264 encodes .m4v/.mp4 not accepted in iTunes It seems like every once in awhile, when using Handbrake to encode a DVD rip to x264 with either the mp4 or m4v extension, I can't add the resulting file to my iTunes Library. When dragging and dropping, the Library window highlights like it's going to accept (If you try to drag an unsupported file, the window doesn't highlight, and the thumbnail you're dragging flies back to the original folder).

If I rip a handful of movies and do an overnight encode session, in the morning some of the files can be added to the library, some can't. I have to re-do the ones that don't work.  Is there a way to found out what the problem is? An iTunes setting that will output what it doesn't like about certain files? A piece of software that can validate the files?  The resulting files play in VLC and Quicktime Player regardless of if they're added to my iTunes Library.
Handbrake keeps individual logs for every encode. You can find them in ~/Library/Application Support/HandBrake/EncodeLogs.

If an encode fails, it should contain any relevant information on why it failed.