Hi,
Kindly make sure all the device are snmp reachable with the new IP from the CiscoWorks Server. However more information will be found by enabling the debugging for Device Discovery and then try to run the Device Discovery again.
To enable the debug for Device Discovery , kindly to Common Services > Device & Credentials > Device Discovery > Discovery Logging Configuration > select the modules you are using for device discovery along with Data Collector, Discovery DeviceInfo , Discovery Framework, Discovery util and run the device discovery again and then send the ngdiscovery.log from CSCOpx\log location.
Also kindly make sure you have make the required changes on the Schedule discovery settings as well since Schedule discovery have its own settings for Device Discovery as compared with Ad hoc Discovery settings.
Here is the good doc link created by Sir Joseph Clarke to understand the Device Discovery and for troubleshooting purposes.
https://supportforums.cisco.com/docs/DOC-9005
Note :- ngdiscovery.log contain information about the IP address of your device in the network.
Thanks,
Gaganjeet