AirPlay doesn't find all computers in the same network

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?