External Hard Drive & iTunes

I have an external hard drive connected to my time capsule. I am saving some of my movies on there, to save room on my iMac HD, and linking them to my iTunes library. Strange thing is, if I haven't watched the movies for a while and I start my apple tv, the movies that are on my external hard drive don't show up in apple tv. They are still in my iTunes library. The movie name is there but no cover. I have to right click on the movie in iTunes, select get info and when that dialogue box come us with the info I click okay and then the movie cover shows. It then shows up on my apple tv!! Does that make sense? All my other movies in iTunes stored on my iMac HD show up, its just the external hard drive movies that keep doing this. Any idea why??