You have to check two things:
If datagatherer is started in the node
and if the agent is up and have discovered the service(s).
In %oraclehome%/network/agent you have services.ora
The services discovered are there. If the file is empty,
check %orahome%/network/log/nmiconf.log to see
the errors.

are you on NT?
Resolution:
stop the agent.
If the dataGatherer was not started there is a registry problem:
The oracle homes are not in order:
e.g. you have home1, home2, but you do not have home0.
After starting the datagatherer, you delete all files
*.q on %orahome%/network/agent/, snmp_ro.ora
and snmp_rw.ora on %%/network/admin and restart the agent.
If datagatherer work and your nmiconf.log have errors, there is a registry problem caused by multiple oracle installation and removing, you better delete all, and oracle repertory on registry and reinstall all oracle stuff.