无法使用Prism从回调方法内部导航

I have a small application using WPF and Prism. I have my shell and two modules. I can successfully navigate between them in the "normal fashion" (e.g from a button click) so I know they are wired up for navigation correctly. However, if I perform some asynchronous operation that fires an event on completion, I can't navigate from inside that event handler. The last thing I tried was using Event Aggregation to publish an event back to the UI thread, but it's still not navigating. The Subscriber to the event gets the event successfully and fires RequestNavigate(...) but the UI doesn't update.I have a small application using WPF and Prism.