iTunes freezes on my PC upon starting many movies

I'm trying to show a movie to my class (I'm a MS/HS teacher) and some of my movies work just fine, but others just get the spinny circular loading wheel in the middle of the screen, and then the wheel just stops spinning as well, and everything locks up. I have to go to task manager to close the program. Any help on why this is happening? I've shown these in class before so I'm not sure why some won't work now. I've purchased them so I'd love to use them for their intended purpose.