1. Check the $ORACLE_HOMEnetworkagentlognmiconf.log for warnings or errors. 
2. Verify the database sid exists in the listener.ora file. When the Agent parses the listener.ora, it looks
for the "sid_list=…" section. If this section is missing, the Agent will not discover the database.
3. Verify the database sid exists in the oratab or Microsoft Registry. (In some cases, the sid is case sensitive.)
4. Stop and Restart the Agent. NOTE: This step is necessary because the Agent will only parse the files during startup.
5. Check the services.ora file and verify the Database services now exists in the file. 
6. Refresh the Node through the EM Console.