I have a class full of trainees staring at me because none are able to preflight their projects. Clicking on the Preflight button in the dashboard produces a progress bar that flashes by in half a second, but the devices report that there is no content. We have preflighted content, collections, layouts... then for good measure published them all as well. It all looks fine in the Layout Templates preview section of the dashboard, but it simply won't show in the AEM Preflight app.
Preflight shows only content that has been added to the top-level collection or to any descendant collection. If you haven't added content to the TLC, nothing will show up in your app--you'll get a "Content not found" message.