MP3 Player Failing to Recognize Compatible Content

My mp3 player (30G Zune) will recognize maybe 3/4 of the movies I [legally] download, but the other 1/4 it just won’t recognize at all.

For example, say I DL’d Movies A, B, C and D. Movies A, B, and C will all appear in my “Videos” collection when I open up the mp3 player software, and I can transfer them to my player. However, for whatever reason, Movie D will not show up in my collection. I can open it and watch it (using my mp3 player’s software) if I open the file in my Downloads folder, but my mp3 player software just refuses to recognize that Movie D is even there.

Anyone know what’s going on?