VAMT 2.0 works with IP address, but is unable to connect to the WMI service when using AD

We're running Windows 2008 R2 servers and (mostly) Windows 7 clients. I'd like to use VAMT 2.0 to push install KMS keys in place of MAK keys. While it works great with about half of our virtual and physical machines, I'm faced with this error on the other cases, when discovering the machines through AD:
Unable to connect to the WMI service on the remote machine

If I try to connect to those same machines with Powershell, I get:
Get-WmiObject : The RPC server is unavailable. (Exception from HRESULT: 0x800706BA)

All my research points to a firewall block, but we're not using any local firewall. And again, the procedure works in half of the cases.

Oddly enough, when I add the machines by their IP address, VAMT is able to reach them. If I add them by host name it is not. I'm clueless.

EDIT: the machines I'm testing with are VM clones. I can ping them and resolve their hostnames from the machine where VAMT is installed.