This behaviour can be experienced when the device filters for the 'Non-SNMP Devices' node group, has not been set correctly or is deleted.
To confirm if this is resulting to the issue, goto the NNMi Console -> Inventory -> Node Groups -> Non-SNMP Devices (Node Group selection)-> Device Filters (tab)
Verify that the default Device Filter: Device Model - <No SNMP> still exists. If not, then re-create this device filter by clicking on the 'new' device filter tab and configure as illustrated below -
Device Category : blank entry (--Choose One--)
Device Vendor : blank entry (--Choose One--)
Device Famity : blank entry (--Choose One--)
Device Profile : <No SNMP>