Hi,
Can you confirm 2 things.
1) Are you using host name or IP address to define NTP server.
2) If using host name can you confirm that you have a proper DNS server defined.
Test to make sure it is working by pinging a URL like for instance www.google.com
Note:
If you know the IP address of the NTP server try using the IP address and running the 'show ntp associations' command again.