Moved span port and appliance up to sit in front of firewall/load balancer. The span port that Foglight Experience Monitor (FxM) had been connected to previously was behind the firewall. Now FxM is connected up to our network on a span port in front of the firewall (and the load balancer) and is exposed to the whole world. But the appliance is not capturing traffic from some servers.
Its likely there are now different servers involved/available on the outside of the load balancer than there was before, when FxM was situated on the inside of the load balancer. And FxM isn't aware of those servers on the outside because it couldn't see traffic from them when you were originally configuring the appliance.
You most likely need to add in a different server set to listen to on the outside of the load balancer. Re-discover the servers to find any possible new ones that can now be seen.
Log into the FxM web console and go to (Configure | Monitoring | Servers) 'Auto-Discover Servers'. This page initiates a process that automatically discovers the IP addresses of servers by monitoring ongoing network traffic. The page will update every few seconds expanding the list of servers as they are discovered.