My movies won't sync into my ipod classic because it says the movie cannot be played on my ipod classic.

I bought 'Tangled' (HD) from itunes store but it won't allow me to sync into my ipod classic. Even after i reduce the 'HD' to 'SD', it still says that the movie can't be played on my ipod classic and it refuses to sync it. What should i do? 😟 And in the future, what do i need to check on before i buy a movie? I want it do sync into my ipod classic. Is there any specs that i need to take note of before i buy any more movies to sync into my ipod classic in the future?