What are the results of using NSLOOKUP from a client to resolve the hostname? Also you have to make sure the port is open on the offload server (usually 9053 I think). The default windows firewall policy on the offload server would probably stop connections.
I had issues with my offload scanners and using domain names and FQDN, agents kept going into a disabled state. In the end I had to setup the offload scanners via IP address. In the policy you can specfy primary and backup and that works for us.
Found the problem.
We created our DNS entry as a CNAME instead of a A record.