Movies won't sync to my iPhone 4

I wonder if anyone can help, I purchased movies on iTunes ages ago and they were on my iPhone 3 with no problems. When I got my iPhone 4 I just assumed that when I did a back up all the stuff that was on my old phone would be on it. However some of the movies just won't sync for some weird reason. I have plenty of memory left. Anyone any clues as to why this has happened.

Thanks