If Windows Discovery fails with valid credentials, verify the service account has local administrator rights and WMI permissions on the target server, ensure network connectivity on required ports (135, 49152-65535) for RPC, check for WMI and RPC service availability on the target, confirm the account isn't locked out, and ensure correct WMI timeout settings. You can test connectivity using PowerShell and Test-NetConnection, and use the WBEMTEST utility to verify WMI functionality independently of the discovery tool.