Here are some of the ways to do a troble shoot on the problem.

First check to see wheter you were able to start the agent.

lsnrctl dbsnmp_status

If you are getting an info saying that the "The db subagent is not started" then follow the following steps.

$ORACLE_HOME/network/log/dbsn*.log

Now see what kind of errors that you see there.

If the logs show NMS-00001

then go to the directory

$ORACLE_HOME/network/admin

mv snmp_ro.ora snmp_ro.ora.old
mv snmp_rw.ora snmp_rw.ora.old

Now go to the directory

$ORACLE_HOME/agent
mkdir junke
mv *.q ./junke
mv services.ora ./junke

now try starting it

lsnrctl dbsnmp_start

Confirm it by dbsnmp_status also confirm through

ps -ef|grep dbsnmp

you should see a parent and a child process of dbsnmp

Once after confirming things, go ahead and delete the .old files and juke directory.

Hope this would help you.

Sam