AirPlay doesn't find all computers in the same network

Question

iMac is connected to Bose speakers and sound works just fine.  I can play music from Pandora, iTunes or any other source.
I can also switch the sound from same iMac to AppleTV or to a MacBook Pro, as iMac finds them all from the AirPlay dropdown list.
However, MacBook Pro doesn't "see" that same iMac.  BTW, both are running Mountain Lion and latest iTunes version.
What am I doing wrong?  Any ideas?

Answer 1

As Barney-15E has suggested, this looks very much like 2 routers, and some systems talking to router A and some systems talking to router B.
Bonjour protocols will NOT cross a router boundary, so systems on router A will only see other systems on router A, and systems on router B will only see systems on router B.
The recommented network setup is to make the router connected to the broadband connection be the ONLY router in the home and configure all other routers so they are in "Bridge" mode. That is to say, the other routers are configured so they just extend the primary router's subnet.
How a home router is put into bridge mode is different depending on the make and model of router.
In your case, I think the Belkin should be the primary router, and you should put the Time Capsule into "Bridge" mode. Applications -> Utilities -> Airport Utility -> Time Capsule -> Internet -> Connection Sharing: Off (Bridge Mode).
Once you put the Time Capsule in bridge mode, all your systems should be on the same Belkin subnet, and now Bonjour will see all the Macs in your home.