I see you purchased the High Definition copy of this movie. The SD version is not downloaded by default. However, you can download an SD copy of a purchased movie at any time. To do this you would need to access your prior purchases via the Purchased section under Quick Links of the iTunes Store. From there you can select the movie that you wish to download. In the bottom right corner of that section, you will see an option that is selected by default called Download HD When Available. If you deselect the option, the SD versions will present themselves and should be able to be downloaded. Once the SD version is downloaded, you should be able to sync the movie with your iPod nano.
I have included an article below that walks you through the downloading past purchase process.
Also, you can attempt to create an iPod version of the movie. The article below will walk you through that process.