IPod movie download help?

I am trying to down load the movie Sucker Punch, but for some reason I can't! I have more than $15 on my iTunes account, which is the price for the movie. And I can only do this on my iPod, not from a computer. When I tap "buy $14.99" then "buy movie", it doesn't ask for my iTunes password or anything! Ugh I hate this so much!! Plz help ASAP :(