The Preflight and Publish workflows are separate and work with specific apps. Both use the same content in your project, but the preflight button works with a preflight app and the publish button works with a non-preflight app.
So if you're wanting to proof your work using the free DPS Preflight app (available in all the app stores) -- you'd install that app on your device, launch it and sign in. Then you'll see your project, tap to look at your project. As long as you've clicked the preflight button in the portal, you should see content. (see caveat below)
The same workflow applies if you've built and installed your own app with "enable preflight" turned on. This would be your "custom preflight app" signed with your own credentials. This app also looks for content through the preflight workflow.
OTOH, if you've built a custom app that does not include the preflight option, this app will ignore the preflight workflow and will only display content that's been published.
You must have content (typically a collection) in the default collection -- If you do not, then nothing will be displayed.
Perhaps this will get you going, otherwise share more details.