GenStage Dispatcher not processing deliver_later events

I cloned the BroadcastDispatcher and am modifying that to distribute events a bit differently. In some cases I want to process one event and return the rest to be processed later. I return the remaining events in the result but dispatch is not called with those remaining events. I have demand waiting to be met, and I expected dispatch to be called again shortly after the one that provides events to be dispatched later. What is required to trigger the dispatch again with those deferred events?