Check out this thread:
UDT Hostnames and IP Address stopped working
both RichardLetts and myself seem to be having the same problem.
Can you read through there and see if you are also experiencing this problem?
I originally asked if it was ever working as this would give us some context to your problem.
UDT relies heavily on arp tables. This is where it gets its data from.
When you run "show arp" command on your core switch do you get results of MAC addresses and their corresponding IP addresses?