A few things might prevent devices from being found via SNMP:
- There may be firewall or access control lists blocking UDP / SNMP traffic to the target device.
- Some devices (like Cisco ASA firewalls) require SNMP client IPs to be white-listed before allowing access. If this is the case be sure the IP of your PC is white-listed before scanning.
- Specific MIB OIDs may not be configured for SNMP read-access in the device.