DNS server is not working - timed out

I have 2 child controllers. But one of the controller can resolve names and the other doesn't. I can't find out the cause. Any suggestions to troubleshoot this?

Edit: It seems like AD_child_02 cannot resolve example.com but it can resolve testserver1.example.com.

AD_child_01 can resolve example.com and it can also resolve testserver1.example.com

AD_child_01 (10.10.10.10) - test1.example.com

AD_child_02 (10.10.10.20) - test1.example.com

When testing with AD_child_01 :

nslookup test_server 10.10.10.10
Server:  AD_child_01 
Address:  10.10.10.10

Non-authoritative answer:
Name:    test_server
Addresses:  10.10.10.30

When testing with AD_child_02 :

nslookup test_server 10.10.10.20
Server:  AD_child_02 
Address:  10.10.10.20

DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
*** Request to AD_child_02  timed-out0