Cannot watch movies I bought through Itunes

I'm glad I gave up on itunes, it was always very difficult to get it to work right with purchased movies, but I still want to be able to watch the movies that I actually bought with itunes.

In any case, I'm trying to watch some of my movies. They were bought with another computer I think, I put them into my itunes library and press play, it asks me to authorize the software, I do that, and it accepts my password and says that 2 out of my 5 machines have been used, but then I just get a blank black screen that does not play a movie no matter how much i fast forward.