See this thread for a potentially helpful discussion:
http://www-10.lotus.com/ldd/nd6forum.nsf/55c38d716d632d9b8525689b005ba1c0/1102d6fc7642b39385256d6600523896?OpenDocument
Also check the signitures on the agent before the agent is enabled, the signitures have to be either Notes Template Developers or someone else with the rights to specify OnBehalf.
For a more detailed discussion of this topic, you might find "decoding ND6 agents" article helpful. You will find a link to it in the Agent FAQ or by searchin article archives directly.