I have recently inherited the vulnerability scanning responsibilities from a security analyst who has left our company.
As I've grown more familiar with the devices being scanned by Nessus I've discovered that none of our IPS devices appear to be picking up on scans. They are set up in Nessus by FQDN and will resolve in Nslookup, but fail to Ping. I guess I'm wondering if this may be a matter of the devices being unreachable on our network, or, more so, if IPS devices cannot be scanned at all in Nessus.
Nessus can scan whatever. That doesn't mean that the Nessus scanner and the IPS systems don't have firewalls between them. Frankly, that is where I would look first (assuming they're not in the same subnet, etc.)
A firewall is probably blocking incoming ICMP traffic. First try tracert then NMAP or wireshark to see what and where traffic is blocked to the host. You may find the device itself is shunning ICMP traffic as its own protection.