2 places to check. The local logs and in the console, are there any “running” actions on those clients? Running tasks have bitten me in the past and they stop reporting because they are downloading or running something.
If you cycle the service on one of them, does it light back up? If you send a refresh to the device, does it show back up? When you send a refresh, do you see the acknowledgment on the endpoint logs? Something like “ForceRefresh command received. Version difference, gathering action site.”