I bought a movie via itunes but i cannot see it. why?

When did you buy, and what did you buy it on (computer, iOS device), and by 'cannot see it' you mean ... ? If you've just bought it then it should have downloaded onto the device/computer that you bought it on - so into the Movies part of your iTunes library if using your computer, the Videos app if using an iOS device.

If it's a film that you bought it in the past then you might not be able to redownload it if you've changed countries since buying it, the rights-holder doesn't allow redownloading, or if it's no longer in your country's store.