This does not make sense.
Switches are L2 devices and do not have IP addresses on their interfaces.
Switches are usually managed through a dedicated Vlan/Interface, not loopbacks (but some switches can be configured with LB interfaces and IPs).
This sounds more like a L3 or FW problem.
As superfly has mentioned, you must be able to ping the switch from the NPM server.
If Orion does not first receive an ICMP echo-reply from the device, it will never try to poll it with SNMP.
If there is a firewall in the path, then you may want to reduce or increase the ICMP payload Orion sends to the device.
This is mentioned in the SW Knowledge Base.