Yep, the agent not running would cause your problem.

On the target machine, do you have something like a OracleSNMPPeermasteragent service? Start that, and then the agent. If the agent starts, try rediscovering.

If the agent doesn't start, go on the target machine, and can you look in $ORACLE_HOME/network/logs (where oracle_home is where you installed the agent) and have a look at nmiconf.log, dbsnmp.log and agntsrvc.log, anything in them?

Bazza