Films keeping stopping when playing on Apple TV from iTunes library on iMac

HI,

we have BT infinity - great wireless bandwidth and download HD films from iTunes and watch via Apple TV no problem whatsoever. BUT when we try to stream our own DVD films stored on our iMac iTunes account through Apple TV it simply doesn't work! The Apple TV device is right next to the iMac as well. Interestingly when we stream from the iMac to the ipad it works just fine so I believe there's a problem with Apple TV.

any ideas?

thanks

Wendy