When a network device is not being found via SNMP. We recommend troubleshooting the issue by installing UVexplorer on a local machine - the same VLAN as the device in question.
If UVexplorer does discover the network device from that location, but it is not discoverable from the previous server (from a different VLAN) then it could be the network device has limits on which hosts can talk SNMP to the device.
Please check your switch to see if there are any restrictions regarding the host that can use SNMP.