It sounds like the Systems Manager agent on the two iMacs itself doesn't have communication with Dashboard.
Curious, were you able to remote-connect to either or both of those iMacs previously, and now it's not working, or it never worked in the first place on those two? And obviously the SM agent has been properly installed on those two iMacs? Perhaps try reinstalling the SM agent and make sure the trust settings are allowed. And you can confirm the agent is actually running on these two devices? Perhaps you also tried un-enrolling and re-enrolling the devices into the SM network?
It could also sometimes be an upstream issue, if the devices for whatever reason cannot communicate with Dashboard. Are they coming from a different VLAN or section of your network where something upstream could be blocking? SM communications do need a handful of ports to be opened for agent communications, but that's usually only if your behind a restrictive outbound firewall. You mentioned rebooting the FW, but also check the logs just to confirm if you're seeing the traffic from the agent on those iMacs getting permitted or denied.
