Maybe it's a matter of bad routing ARP requests? :p
(Or a problem with your router's DHCP)

(client tools were enabled)

FROM: http://technet.microsoft.com/en-us/library/jj680093.aspx

Maximum components for CLIENTS:

Virtual network adapters = 12
- 8 can be the “network adapter” type. This type provides better performance and requires a virtual machine driver that is included in the integration services packages.
- 4 can be the “legacy network adapter” type. This type emulates a specific physical network adapter and supports the Pre-execution Boot Environment (PXE) to perform network-based installation of an operating system.

I'll think about what that means next. I do have 4 virtual machines currently running. 2 Windows though! I'll test by shutting down 1 virtual client, see what that gives :s

Maxima for Server running Hyper-V:

Running virtual machines per server = 1024
- Virtual switches
Varies; no limits imposed by Hyper-V.
The practical limit depends on the available computing resources.
- Virtual network switch ports per server
Varies; no limits imposed by Hyper-V.
The practical limit depends on the available computing resources.

Devvie

When defining Hyper-V's networking properties, you can create multiple Virtual Switchs (so you can have separate VLANs), you can try appending the new machine to a new Virtual Switch and connecting that switch to the server's main connection

I'll study this next: http://www.serverwatch.com/server-tutorials/harnessing-the-power-of-hyper-v-network-virtual-switches.html