problems with iTunes. again.

hey all

I have another iTunes query, i seem unable to stream stuff from my iMac to my ttv using my mba. if i go to the desk and use iTunes on my iMac there is an icon to stream to my tv. if i use my MBA to play a local file I can stream it. but i can't manage to stream from the the iMac to the atv whilst using iTunes on my MBA. it is a fairly pointless thing to do, but if i want to watch something on my tv that is stored on my iMac do i have to use the iMac to stream it?