John Marlin [MSFT] wrote a very useful article on how to troubleshoot when a node is removed from active Failover Cluster memberships. As Hyper-V uses Failover Cluster for clustering technologies these tips also apply to Hyper-v Clusters. Read the full article on the link below.

http://blogs.technet.com/b/askcore/archive/2012/02/08/having-a-problem-with-nodes-being-removed-from-active-failover-cluster-membership.aspx#phrss