The most common reason for this would be that the SNMP agent you've deployed on your GNU/Linux system is running the default/maximum security config. So it is running but provides only the very basic data (i.e. responds only to requests to system part of the tree, like sysObjectID, sysDescr, sysUpTime etc.).
Aren't you running for example Net-SNMP agent on that box? This one is often by default using 'paranoid' priv level which is essentially what I've described above.
If that's the case you can edit the snmpd.conf to setup a community string with access to other/all parts of the MIB tree so Orion is able to obtain the information it needs for monitoring of the various resources. Can help you with that if it's e.g. Net-SNMP on Debian.
There are more possible reason but this is by far the most common.
Please follow the KB and this issue will be resolved. You have to allow SNMP to reach all the OID in order to access resources .
Let me know if this helps, or if you require any further information.
Thanks very much for the input.
We'll give this a try next week & I'll feedback once it's done.