There should be an alert for when a node does not return data when polled but the node is up. I had an issue where several Windows servers were not responding to SNMP requests from SolarWinds. This had been going on for months. The nodes were all seen as 'Up' by SW since the responded to ICMP but for one reason or another, they were not providing data to the poller. In one instance, the Windows SNMP service hung and needed to be restarted. In another, there was a know issue with the OS where it would respond to SNMP requests from a different IP address if the NIC was configured with more than 1 IP. In both cases, the server was up but not providing new data like drive space.
There are some default alerts for when the node has not been polled, or when the polling engine has not updated the database, but even tech support said that an alarm to alert when SNMP requests are not responded to even though the device is up does not exist.