This is most often because of a firewall on the network you are scanning. Port scans and OpenVAS vulnerability scans both make many requests to the target IP address and some firewalls will detect these scans and start blocking the requests.
You can set up a firewall rule to allow traffic from our scanners. See https://help.hostedscan.com/help/what-ip-addresses-are-used-for-the-hostedscan-vulnerability-scanners for more information.