Unable to watch 3 movies I rented. Please help!

I rented 3 movies through iTunes to watch on a trip I went on. While on the airplane I went to watch them on my iPad through the videos app and it told me none of the movies I had rented could open. Does anyone know why this happens? During my layover I was able to watch a movie because my iPad was connected to wireless, but I thought you could still watch them without wireless. If anyone has any advise on this please let me know. I have paid and been unable to watch 4 movies so far on my iPad. Thank you!