Apple TV doesnt network all devices can share

I finally swap over and have gone 100% Apple.

Now have a iMac, along with 2 apple TV's and numerous iphones and ipads.

Since moving to iMac, the apple TV's will not see that the computer library is on the network. apple ID etc all correct. Network works and tested.

The iMac can share across the network to all other devices. i.e. can go to video and share llibrary on ipad and see required videos.

The old Dell laptop can still stream to apple TV's successfully, the apple TV's still recognises that computer and its library.

I get the "make sure the computer is on the same network as your apple tv" message.

any ideas? I have turned off and on home sharing, the router / wifi, apple TV and iMac.

cheers