Polling rate or more weight on your poller - have frequent maintenance schedules to keep your poller healthy - monthly server reboot + remove unwanted monitors from your system.
If more number of monitors are unknown try restarting the services for sure you would see the difference if this works then we can dig deep into the issue that you are talking about.
Good, solid communication between the SolarWinds system / poller and the targets is key to SAM. It's possible that your poller is overwhelmed by the number of devices. If you were to remove a number of devices from SAM and the remaining systems start showing up again, you may need to invest in another poller and split the load. Another possibility is the polling frequency. I don't recall the default polling interval (9 minutes?) but if you increase that timer you may be able to get all of your devices responding again.
Last, you may have a network issue where the SNMP traffic is being blocked by a firewall or by an ACL. Obviously, that will prevent SAM from reporting on those devices.
