Streaming on one W8 box not the other

This is driving me mental
I have two Windows 8.1 boxes.
The first is connected to the ethernet to the router, from the router then to my Denon amp. I can successfully see the Amp in Media Devices and even use AirFoil to stream from other applications to the amp.

But, I have a second box on Wifi (no ethernet) which is on the same homegroup but cannot see any amp listed in media devices.

Is there some thing I am missing here? Config between network adapters or something? It picked up the amplifier previously but it has now dissapeared and even AirFoil is not picking up the amp!

Firewall would be the first thing that I would be looking at on the troublesome computer. As long as you are on a router doing NAT. There is no need to have the firewall active on a computer on your own LAN.

thanks
I disabled that as part of my testing. didnt work.
I've checked too that the PC is part of the same workgroup and subnet etc.

any other ideas?