As i crashed my head on it for over two hours yesterday i'll just drop a note here.
On (the latest?) Debian releases SNMP is by default limited to localhost queries.
The standard config file /etc/snmp/snmpd.conf has no point where this limitation appears.
The limit is in the /etc/default/snmpd file where under SNMPDOPTS you'll find a 127.0.0.1 at the end of the string. removing this will enable access from everywhere, not sure if it can be used for limiting to a particular network, didn't try... i went for the firewall
Hope this helps