IPs that are not getting discovered event though you may have the credential in the discovery credentials table and are validated.
Release or Environment
Encrypted serial number / IP which is used in the discovery process.
Encryption of the CI can be seen by examining the XML of the record.
If you see encryption on the IP or serial number which is used in the discovery process then this will fail at this step.
You can decrypt the IP / serial number then retry the discovery process.