First off... is the VM powered on?
This is important as often powered off VM's will not replicate, and thus you get an RPO violation...
Replicating Virtual Machines
You can configure replications for powered-off virtual machines, but the data synchronization begins when the virtual machine is powered on. While the source virtual machine is powered off, the replication appears in Not active status.
If the VM is powered on... Check to see when the last sync was successful, if it was recently then leave it and watch it a little longer...
If it was a long time ago try vMotion it to a different host... sometimes that will get things moving...
Check the vmkernel.log and look for HBR. cat /var/log/vmkernel.log | grep HBR
That should give you the next clue