Stacking
A few of my systems do not join the stack. What should I do?
Check if all servers are running the same software version. If it's a VM-based environment, check if all systems have a unique Serial Number. Stacking multicast IP and the port has to be identical on every system and enabled on the same network.
If all servers are visible in System -> Devices, but you are failed to join them to stack, you can try to export the configuration of the existing stack, perform the factory reset on the machines, reconfigure or import only network configuration and try to join additional systems. After all the servers are joined, import the system configuration. This resolution may work in cases when the network is slow and the configuration file is very large.