Try the different versions of NTP. Here is how you change the ntp version
in configuration mode
switch(config)#ntp server (ip address of ntp server) version (choose from 1-3)
Try them all and see if it will synk with the server
check your IOS is not lesser than 12.1(6)EA2c.
Also check whether you are able to ping the NTP server from the switch.
Use the
debug ntp packet
debug ntp valid
to troubleshoot the ntp packets.