You need ‘tcpdump’ on each box/VM. This tool will show you the network traffic, so the basic steps are:
tcpdump on the VM/box where the app runs.
then you will see if doing any traffic or not.
if not then the issue is within the app.
if yes, go to the next hop
tcpdump on the next box/VM (sys-firewall, sys-net - depends on your virtual network topology.)
tcpdump here to see if your traffic pass by…
if not, then something blocking on this machine
if yes, go to the next hop
repeat until you found where your traffic stops.
If oyu have a woring one, that a good start to see how it is goes if working… then you can compare.
Sure, this assume some basic networking knowledge, and to know how to use tcpdump.