Why won't LJM open devices or find devices via LJM_ListAll through the network?

In general, you may want to consult the Basic Network & Troubleshooting guide.

You may want to try LJM Specific IPs for devices that can be ping'd but for which LJM cannot seem to find.

Linux

Check your iptables configuration. If disabling iptables fixes the issue, you may want to alter your iptables rules. In the general case, running a command such as:

sudo iptables -I INPUT 2 -p udp --sport 52362 -j ACCEPT

will add an iptables rule in the second spot (INPUT 2) to allow UDP packets on the port 52362, which is the UDP port LJM uses to discover devices.

Don't forget to save your iptables after altering the configurations. For example, on CentOS:

sudo /sbin/service iptables save

You may want to test your iptables is correct after a reboot.