WHY do some .MP4s NOT appear in list of movies?

I've got both .VOB (ripped DVD) and .MP4 (h.264 ripped Blu-Ray). On my Sony SMP-N100, ALL of the .VOBs appear, but SOME of the .MP4s are missing (authored from same program).

Windows' Media Player plays the "missing" files just fine.

Any thoughts?

p.s. I'm using the "Sony BD Player" profile with the SMP-N100.

[Cerberus]

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