You're going to want to start with one machine to troubleshoot as it's likely the same cause, or a small few. I would start with checking:
- The LEM license is active and has additional nodes (you aren't maxed out).
- The nodes have port connectivity to the LEM.
- The nodes have the correct IP/Hostname configuration for the LEM (they can ping it and that is what matches in the spop.conf file).
These are the general things I would spot check before moving into the agent logs and taking a look at the errors there. Additionally, you may find this useful with some additional detail:
Troubleshoot LEM agent connections, 64-bit - SolarWinds Worldwide, LLC. Help and Support