What do I need to do so that the agent rescans an event log?

EventSentry is an event log monitoring application, and does, by design, not rescan the event logs. It monitors the event logs and processes current and new events in real time. It does not process past events.

The only exception is when you have the "Bootscan" feature activated. When this feature is active, EventSentry will rescan the event log after a machine has been rebooted. However it is not currently possible to force the agent to rescan a particular event log.