A firewall rule or related ACL, perhaps? I would check that first. Are you actually seeing the ICMP requests reach the servers, i.e. via network analyzer such as Wireshark? It could be that the firewall itself is responding to the PC’s ICMP requests rather than the servers. A good quick test is to use a tool such as SoftPerfect Network Scanner and run an ICMP sweep of the entire network subnet. If all IP addresses respond quickly (and, especially, without any hostname information) your PC’s ping requests are likely just stopping at the firewall creating a false positive.