Secondary DNS / Domain Controller doesn't seem to be functioning correctly.

I recently have had an issue with my primary domain controller/dns server's hard drive space has filled up and the primary domain controller stops working.

However, when this has happened my secondary domain controller / dns server doesn't seem to be doing name resolution.

When this happens some users can no longer use RDP to get to a server by server name. They can still rdp by ip address, just not by name.

I will soon be addressing the hard drive space issue, but in the meantime it seems that my secondary dns server is not functioning the way I think it should.

Any suggestions or things I should try?

Thanks,

James