Essentially, I have a base (empty) scene that only contains UI, and several other scenes that get loaded which contain the AR content (including the ARCamera).
When I load the first scene, it tracks no problem. As soon as I load another scene, it won't track. Keep in mind that each time I load a scene I'm creating an ARCamera, so this may be part of the problem, but I don't know. It all works fine in the Unity editor, but not on a device.
And note that whichever scene I load first, it works fine. But whenever I move to another scene, it no longer tracks.
Hope this makes some kind of sense.