Applications :: Final Cut Pro Keeps Freezing When Try To Render?

Ok when I render at all it never EVER works. It has worked a couple times but recently I do everything people suggest and it doesn't work. I've converted both into the same format so it'll work, I've restarted final cut pro, I've restarted the computer, I've repaired disk permissions, i've done render all. All worked at certain points but for the rest it doesn't work. What am I doing wrong and how can I fix it?