This often happens when the target does not respond to ICMP ping requests and the scanner believes that the host is down, thus it does not attempt to do the port scan at all.
In order to fix this, you should rescan with Nmap’s option (
-Pn) enabled. This option (also called “Don’t ping host“) tells Nmap to no longer try to determine if the host is alive, but just start scanning for open ports.
The same applies for the Network Vulnerability Scan with OpenVAS, you should disable the “Check if host is alive before scanning” option.