DNS Problem

i have 2 integrated active directory DNS servers , preferred is x.x.x.y and alternative is x.x.x.z when resolving with nslookup it always points to y server.
when y server go down, clients and nslookup command still points to y server and reply with query refused and not pointing to z server.
if i used nslookup server z it answer and all clients start using z server as DNS server.

How can i make them switch automatically?

 

On Z server where is it pointing for DNS?