Our secondary DNS server has been synchronising perfectly with our primary DNS until about a month ago. Since then it no longer adds domains to both servers.

Can anyone advice how to get the DNS servers synchronising on NT Server 4.0 please?

As much advice and assistance as possible would be most appreciated.

Did you check the event viewer?
If there's an error then that's where you'll find some more info about what's wrong

No DNS errors. Sorry.