After configuring a scan job an expected Device was not present in the IT Explorer results after the scan job was completed.
Consult the Scanlogs
Go to the below page and double click the scan job row that was expected to contain the missing device.
The below window will then pop up.
Find the the row with IP address that was configured for the missing device, if it is present.
If it is not present then the Appliance was unable to communicate with the target in any way.