My guess is that either the servers on the other subnet do not allow incoming ICMP traffic or network firewall rules block ICMP between subnets.
If it hangs for a while, the network firewall rules are probably also dropping packets for certain ports (probably most all of them). -- which would make a Nessus or nmap scan take forever.
To see if that's the case, try nmapping only a single port that you know is accessible -- like port 80:
nmap -p 80 -P0 <IP_ADDR>