I seem to recall a problem on 64-bit platforms where 64-bit SNMP request IDs were being rejected by libsnmp because it parsed them as 32-bit IDs, but only if the values were large enough. In this situation, I was able to probe the OIDs just fine using snmpget, but devmon failed to get the responses, but only sometimes. Comparing packet captures of failures and successes showed the problem.

A previously posted thread might be of some assistance:

http://sourceforge.net/p/devmon/mailman/message/30148661/