RHEVM: Monitor - Hypervisors "down"

Have a cluster with two RHEV hypervisors.
Looking in the RHEVM GUI monitor page these are reported as down (A red arrow in the graphs, which results in no percent report on the CPU,Mem or Network utilization).

They have status UP in the cluster and have running VMs on them with no issues.

Why is the monitor not working for these hosts?
Any tips on where to start debugging this would be great.