HA on FortiGate-VM under Hyper-V

Hello

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.

Any solution to configure the HA cluster under Hyper-V? What configuration should I apply both in Hyper-V and in the forti?

Thanks
Regards