Q: I configured DHCP Failover using Windows Server 2012, but my scopes lost contact with the partner server--how do I restart replication?
A: First, check that the two DHCP servers can communicate--one way to do so is by using the PING command. After you're sure connectivity has been restored between the boxes, the easiest way to resolve the situation is to force a replication from Windows PowerShell.
On one of the DHCP servers, open an elevated PowerShell session (it should say Administrator on the title of the PowerShell window). Run the command below to force a replication on all replicated scopes:
Invoke-DhcpServerv4FailoverReplication
The command will show the scopes that have been replicated and the replication status should now as normal. If there are still problems, look at the event log under Applications and Services Logs\Microsoft\Windows\DHCP-Server\Admin and look for Errors.
A common problem I have seen is a time synchronization problem between servers, which causes replication to fail. Running "net time /set" will force a time synchronization to the domain controller.