VR Preview Not Working - Am I missing a setting? [Vive / SteamVR]

I have a project that I am trying to use with a Vive headset but when I select VR Preview, in the headset, I only see the Vive default background.
While the app is running, I can see on my computer monitor that the headset is controlling what I see on the monitor. I can move the headset around in the scene like a camera but it only shows on my computer screen.

When I first hit play, in the headset I see “Playing Next: [APP_NAME]” but it just disappears after a second.

I have Direct Display on. I set the project to build for Mobile and 3D/2D scaled. Fixed Framerate and smoothing is unchecked.

If I start a brand new project the VR Preview works perfectly but I need this project to work.

Does anyone know if I am missing a setting?

A last possible clue is that while the app is previewing, the title of the app in the SteamVR window flickers on and off many times per second.