This is one that had me stumped for a while. No matter how hard I tried I could not get the NETLOGON service in Windows Server 2003 R2 to register it’s SRV records with the DNS server.
In the end it was because "Register this connection’s addresses in DNS" was not enabled. I found this quite strange because I thought that just affected the server’s A and PTR record registrations, but it turns out that it also affects whether the server’s SRV records are registered. I think this is a change to the behaviour in earlier versions of Windows Server because I don’t remember having this problem before.
How you stop the A and PTR records being registered while still enabling the SRV records to be registered is anyone’s guess. I read that you may be able to configure a group policy to prevent the A and PTR records being registered, but I’ve no idea if that actually works.
So anyway, if you are struggling with your SRV records, go to your network adaptor’s TCP/IP settings, click on "Advanced" then the "DNS" tab and make sure this tick box is set: