zSnmpMonitorIgnore is set to false, both directly on the device and on the "folders" that they reside in. If I select Model Device..., it will not error out or complain about not being able to poll SNMP.
I increased the timeout from 1 second to 3 seconds. I'm not sure where the delay is being incurred because when I run the snmpwalk command from the command line, the results are returned immediately. Either way, increasing the timeout duration for the devices in question appears to have "fixed" the problem. Thank you.