I'd say, a lot of the time, people try to play a format the Moment just can't play.
If it is a format it can play, but still won't play, I would say 9 times out of 10, it is the wrong resolution.
It has to be right.
1 point too much and it won't play it.
The phone doesn't have the ability to "make it fit" it is either right, or it isn't.
so before you put them on your phone, make sure you resize them appropriately.
as a side note, I have also see people have problems with videos that are too small too. I can't confirm it, but from what I've noticed, it has to be an even multiple of 800x600.
that means it won't play it if it is 4768300.
you can't multiply that by anything to get 800x600. so when the phone attempts to play it, it doesn't know where to put anything.