Why only sometimes does movies on Apple Tv show the buy option, I need it there all the time?

Two reasons:

The movie companies control what Apple can sell/rent - sometimes they simply change it to purchase only or rental only.

Secondly, if your Settings for Itunes store are set to HD sometimes you won't see an option to purchase or even teh movie at all if it's only available in SD.

There are also sometimes discrepancies between iTunes on computer and AppleTV implying the stores work slightly differently or have different connections. Check if it's available in iTunes.

AC