Device is not reachable: Make sure the device is up and running and is reachable via ping.
SNMP is not enabled: NCM can discover only SNMP enabled devices, so make sure that SNMP is enabled for the device.
Wrong credentials: Make sure the selected credential profile applies to the device you are trying to discover.
SysObjectID not present in NCM: If all the above mentioned criteria's doesn't apply to the device you are trying to discover, then please check the list of SysObectID's supported by NCM under Settings > NCM > SysObjectID Finder.
Click on Add and provide the device ip address and other required information and click next. NCM will find the SysObjectID for the device and you can assign it to appropriate device template.
You can get more information about adding SysObjectID and their usage here
If you are still having trouble discovering your device(s), please contact NCM support team.