Openvpn – DNS Does Not Register at Off-site Locations

First of all, let me give you the specifics of our setup:

• Windows Small Business Server 2008 Domain w/ all applicable updates on the DC
  • The DC does DHCP for the main site
  • The DC does DNS for all sites
• 3 sites including our headquarters where the DC is located
  • All sites are connected through OpenVPN SSL tunnels terminated by an Untangle box at each site
  • The 2 remote sites us the Untangle box as a DHCP server for their subnet, which assigns the DC as the primary DNS server
• Collection of Windows XP and Windows 7 workstations connected to the domain

Here's the issue:

All of the workstations at the main site register with the DNS server on the domain controller fine. As they grab an IP from the DHCP server, it updates the DNS server with the new host record. I have 2 systems (each at different remote sites) that fail to register with the DNS server. I've attempted the following troubleshooting steps:

• Confirmed the network adapter is using the DC as a DNS server
• Confirmed 2-way traffic is possible between DC and workstation
• Verified the "Register with DNS server" setting was checked in the adapter properties
• Attempted ipconfig /registerdns and received no errors

For the time being, I have setup a DHCP reservation for these systems and manually created a host record. This seems to work fine, but I need a solution for any new systems that go out there.