I can't put a movie (from iTunes Store) onto my iPod Nano 7th gen. How do I fix this?

I downloaded 3 HD movies from the iTunes Store. 1 of them will upload onto my Nano 7th gen without any problem. The other 2 will not. I get a message saying that they are not compatible. The 3 movies are all in the same format, and are all HD. I am under the impression that iTunes Store should have given me a SD version of each one as well, and that iTunes would automatically upload that version to my Nano. It is still not working! How can I fix this?