I use my 360 as my main device now and it can be temperamental over files

for my older files I couple of things which have helped me to get them to play

If it's an MP4 file, rename it to M4V and vice versa (doesn't work on all but this fixed a few for me)
Go into properties of the file in Mezzmo and look at the video tab, under the preferred audio option if it shows two items, select the second one, the 360 will now see the file as incompatible and will play it instead of giving an error (transcodes it)