HA on FortiGate-VM under Hyper-V

I' m trying to deploy an HA pair of FortiGate-VM appliances under Hyper-V. Standalone they work fine, but as soon as I change HA mode to a-p or a-a, they lose network connectivity on everything except the cluster management port(s), and the cluster never forms. Changing HA mode back to standalone instantly restores connectivity. MAC addresses don' t appear to change, ARP works, but intermittently. I tried all kinds of virtual switches and vNIC settings, but nothing seems to help. I' ve reproduced the issue using build 5.0.6 on Windows Server 2012, and 5.0.7 on Windows 8.1, different host hardware as well. Am I missing some setting that must be configured to make it work?