Hello,
i Install The Dude (very difficult because missing the information in documentation that work well ONLY if connect to a Read Only account on routers, not with Admin account, no problem) and select to connect to computers on the network (bi-directional firewall full access between Dude and Servers).
The Dude it is capable to connect to SNMP to 20 servers but failed SNMP to 2 servers.
I triple check the SNMP settings and firewall rules and it simply does not want to access SNMP on this 2 machines.
I install another software on neighbor IP and scan al servers and it is working very well on SNMP on ALL servers.
I exhausted my idea of investigation this.